The basic APIs for the CRL Reloading features.
This adds external types to represent CRL Providers, CRLs, and
CertificateInfo.
Internally we will use `CrlImpl` - this layer is needed to hide OpenSSL
details from the user.

GRFC - grpc/proposal#382

Things Done
* Add external API for `CrlProvider`, `Crl`, `CertInfo` (`CertInfo` is
used during CRL lookup rather than passing the entire certificate).
* Add code paths in `ssl_transport_security` to utilize CRL providers
* Add `StaticCrlProvider`
* Refactor `crl_ssl_transport_security_test.cc` so it is more extensible
and can be used with providers

This adds the directory reloader implementation of the CrlProvider. This
will periodically reload CRL files in a directory per [gRFC
A69](grpc/proposal#382)

Included in this is the following:
* A public API to create the `DirectoryReloaderCrlProvider`
* A basic directory interface in gprpp and platform specific impls for
getting the list of files in a directory (unfortunately prior C++17,
there is no std::filesystem, so we have to have platform specific impls)
* The implementation of `DirectoryReloaderCrlProvider` takes an
event_engine and a directory interface. This allows us to test using the
fuzzing event engine for time mocking, and to implement a test directory
interface so we avoid having to make temporary directories and files in
the tests. This is notably not in `include`, and the
`CreateDirectoryReloaderCrlProvider` is the only way to construct one
from the public API, so we don't expose the event engine and directory
details to the user.

---------

Co-authored-by: gtcooke94 <gtcooke94@users.noreply.github.com>

)

This adds the directory reloader implementation of the CrlProvider. This
will periodically reload CRL files in a directory per [gRFC
A69](grpc/proposal#382)

Included in this is the following:
* A public API to create the `DirectoryReloaderCrlProvider`
* A basic directory interface in gprpp and platform specific impls for
getting the list of files in a directory (unfortunately prior C++17,
there is no std::filesystem, so we have to have platform specific impls)
* The implementation of `DirectoryReloaderCrlProvider` takes an
event_engine and a directory interface. This allows us to test using the
fuzzing event engine for time mocking, and to implement a test directory
interface so we avoid having to make temporary directories and files in
the tests. This is notably not in `include`, and the
`CreateDirectoryReloaderCrlProvider` is the only way to construct one
from the public API, so we don't expose the event engine and directory
details to the user.

---------

Co-authored-by: gtcooke94 <gtcooke94@users.noreply.github.com>

)

This adds the directory reloader implementation of the CrlProvider. This
will periodically reload CRL files in a directory per [gRFC
A69](grpc/proposal#382)

Included in this is the following:
* A public API to create the `DirectoryReloaderCrlProvider`
* A basic directory interface in gprpp and platform specific impls for
getting the list of files in a directory (unfortunately prior C++17,
there is no std::filesystem, so we have to have platform specific impls)
* The implementation of `DirectoryReloaderCrlProvider` takes an
event_engine and a directory interface. This allows us to test using the
fuzzing event engine for time mocking, and to implement a test directory
interface so we avoid having to make temporary directories and files in
the tests. This is notably not in `include`, and the
`CreateDirectoryReloaderCrlProvider` is the only way to construct one
from the public API, so we don't expose the event engine and directory
details to the user.

---------

Co-authored-by: gtcooke94 <gtcooke94@users.noreply.github.com>