Collect HTTP transfer stats

At present, when the GIT_CURL_VERBOSE environment variable is set to
a value such as "1" or "true", Git LFS client outputs some of the details
of each HTTP request and response it makes to its standard error stream.

We gather the HTTP request headers to be logged in the traceRequest()
method of the Client structure from our "lfshttp" package.  To gather
the headers, the traceRequest() method uses the DumpRequest() function
from the from the "net/http/httputil" package in the Go standard library.

Our use of the DumpRequest() function for this purpose dates back to
commit 6fafade of PR git-lfs#366 in 2015,
when it was first introduced into what was then the traceHttpRequest()
function in our "lfs" package.  This function gradually evolved into the
current traceRequest() method through a series of PRs.  First, portions
of the "lfs" package were migrated into a new "httputil" package in
PR git-lfs#1226.  That package was later removed in PR git-lfs#1846 after a new
"lfsapi" package was added in PR git-lfs#1794 with a version of the
traceRequest() method very similar to the current one.  Finally, in
PR git-lfs#3244 we moved the method into the current "lfshttp" package.

Throughout these changes, however, our use of the DumpRequest()
function has remained the same.  However, since the Git LFS client only
acts as an HTTP client and not an HTTP server, we should instead be
using the DumpRequestOut() function from the standard "httputil" package
to gather the headers of an HTTP request for logging purposes.

Replacing our use of the DumpRequest() function with the DumpRequestOut()
function would allow us to output a number of request headers that we
currently fail to log, as was noted with respect to the Accept-Encoding
header in issue git-lfs#6193.

Since Go version 1.6, the "httputil" package's documentation has stated
that the DumpRequest() function "should only be used by servers to debug
client requests", and that the DumpRequestOut() function "is like
DumpRequest but for outgoing client requests":

  https://pkg.go.dev/net/http/httputil@go1.6#DumpRequest
  https://pkg.go.dev/net/http/httputil@go1.6#DumpRequestOut

However, at the time we first adopted the use of the DumpRequest()
function, the Go documentation did not include these directives, and
merely noted that "DumpRequest returns the as-received wire representation
of req, optionally including the request body, for debugging" and that
"DumpRequestOut is like DumpRequest but includes headers that the
standard http.Transport adds, such as User-Agent":

  https://pkg.go.dev/net/http/httputil@go1.4#DumpRequest
  https://pkg.go.dev/net/http/httputil@go1.4#DumpRequestOut

It is reasonable to assume, therefore, that when PR git-lfs#366 was written
its authors simply assumed that the DumpRequest() function was sufficient,
given the state of the Go documentation at that time.

As we would prefer to log a more complete set of client HTTP request
headers, though, particular the Accept-Encoding header which the
Go "net/http" package automatically adds to outgoing requests, we
now update our traceRequest() method to use the DumpRequestOut()
function.

We then also update our Go language TestVerbose*() test functions
to confirm that the Accept-Encoding request header is now captured and
reported when verbose logging is enabled.  Note that without the change
in this PR to the use of the DumpRequestOut() function, these revised
tests would fail, since the Accept-Encoding header would not appear in
headers returned by the DumpRequest() function.

At present, when the GIT_CURL_VERBOSE environment variable is set to
a value such as "1" or "true", the Git LFS client outputs some of the
details of each HTTP request and response it makes to its standard
error stream.

We gather the HTTP request headers to be logged in the traceRequest()
method of the Client structure from our "lfshttp" package.  To gather
the headers, the traceRequest() method uses the DumpRequest() function
from the from the "net/http/httputil" package in the Go standard library.

Our use of the DumpRequest() function for this purpose dates back to
commit 6fafade of PR git-lfs#366 in 2015,
when it was first introduced into what was then the traceHttpRequest()
function in our "lfs" package.  This function gradually evolved into the
current traceRequest() method through a series of PRs.  First, portions
of the "lfs" package were migrated into a new "httputil" package in
PR git-lfs#1226.  That package was later removed in PR git-lfs#1846 after a new
"lfsapi" package was added in PR git-lfs#1794 with a version of the
traceRequest() method very similar to the current one.  Finally, in
PR git-lfs#3244 we moved the method into the current "lfshttp" package.

Throughout these changes, however, our use of the DumpRequest()
function has remained the same.  However, since the Git LFS client only
acts as an HTTP client and not an HTTP server, we should instead be
using the DumpRequestOut() function from the standard "httputil" package
to gather the headers of an HTTP request for logging purposes.

Replacing our use of the DumpRequest() function with the DumpRequestOut()
function would allow us to output a number of request headers that we
currently fail to log, as was noted with respect to the Accept-Encoding
header in issue git-lfs#6193.

Since Go version 1.6, the "httputil" package's documentation has stated
that the DumpRequest() function "should only be used by servers to debug
client requests", and that the DumpRequestOut() function "is like
DumpRequest but for outgoing client requests":

  https://pkg.go.dev/net/http/httputil@go1.6#DumpRequest
  https://pkg.go.dev/net/http/httputil@go1.6#DumpRequestOut

However, at the time we first adopted the use of the DumpRequest()
function, the Go documentation did not include these directives, and
merely noted that "DumpRequest returns the as-received wire representation
of req, optionally including the request body, for debugging" and that
"DumpRequestOut is like DumpRequest but includes headers that the
standard http.Transport adds, such as User-Agent":

  https://pkg.go.dev/net/http/httputil@go1.4#DumpRequest
  https://pkg.go.dev/net/http/httputil@go1.4#DumpRequestOut

It is reasonable to assume, therefore, that when PR git-lfs#366 was written
its authors simply assumed that the DumpRequest() function was sufficient,
given the state of the Go documentation at that time.

As we would prefer to log a more complete set of client HTTP request
headers, though, particularly the Accept-Encoding header which the
Go "net/http" package automatically adds to outgoing requests, we
now update our traceRequest() method to use the DumpRequestOut()
function.

We then also update some of our Go language TestVerbose*() test functions
to confirm that the Accept-Encoding request header is now captured and
reported when verbose logging is enabled.  Note that without the change
in this PR to the use of the DumpRequestOut() function, these revised
tests would fail, since the Accept-Encoding header would not appear in
headers returned by the DumpRequest() function.