The ServiceProfile route-matching system is based on recursively nested
condition structures, while the HTTPRoute route-matching system is based
on a list of flat match rules. Therefore, we don't want to use the same
representation in the proxy to match both HTTPRoute- and
ServiceProfile-generated outbound Services, but we would like the same
API to be able to configure both.

This PR builds on #165 by allowing the `OutboundPolicies` RPC service to
return either ServiceProfile-based or HTTPRoute-based HTTP route
matching rules. The `HttpRoute` protobuf message now contains a `oneof`
that's either a list of HTTPRoute rules *or* a ServiceProfile match
condition. This way, the policy API server can tell the proxy about
policies configured by HTTPRoutes or by legacy ServiceProfiles.

The rest of the protobuf is unchanged, as the response classes,
retryability, and timeout fields are the same; only the route matching
condition varies. Currently, the set of arbitrary metrics labels is
included only when the route is configured by a ServiceProfile, because
we have yet to determine whether the new HTTPRoute-based client policy
design will include similar arbitrary sets of metrics labels. However,
this can be moved to the main message type if this is going to be
included in both cases.

PR #165 adds a proposal for the new `outbound` API for discovering
client-side HTTPRoute policy. However, this branch cannot currently be
used by the proxy, because the Rust crate for the proxy API bindings
doesn't actually include the generated code for the new `outbound`
proto.

This branch adds a module including the generated outbound API in the
Rust crate so that the new API can be used.

This branch adds functions to the `linkerd-proxy-client-policy` crate
for converting the `linkerd2-proxy-api` outbound policy protobuf types
into the proxy's internal representations. These are not used in this
branch, but will be used in subsequent changes that actually wire up
outbound policy discovery.

This change was factored out from #2265, so that it can be reviewed
independently of the rest of the client policy discovery plumbing.

Note that this branch changes the `linkerd2-proxy-api` dependency to a
Git dep on the outbound policy branch (linkerd/linkerd2-proxy-api#165),
as the new APIs have not yet been published.

To understand whether the changes proposed in #165 will be suitable for
extension, this change adds a GrpcRoute type. This motivates a number of
followup changes, all of which make the API more closely mirror the
proxy's representation:

* Filter and Distribution moved into HttpRoute. Each protocol/variant
  may have its own filter types, so we want to preserve namespacing.
* Weights moved up into the Distribution
* HttpRoute and GrpcRoute each get a RouteBackend that holds
  protocol-specific filter kinds.
* Queue extracted from Backend. We could use it elsewhere...
* Extracted an `EndpointDiscovery` strategy from `BalanceP2c` so that we
  can change that API if desired.
* Added a `Metadata` onto the backend -- the controller is going to have
  populate that.

To understand whether the changes proposed in #165 will be suitable for
extension, this change adds a GrpcRoute type. This motivates a number of
followup changes, all of which make the API more closely mirror the
proxy's representation:

* Filter and Distribution moved into HttpRoute. Each protocol/variant
  may have its own filter types, so we want to preserve namespacing.
* Weights moved up into the Distribution
* HttpRoute and GrpcRoute each get a RouteBackend that holds
  protocol-specific filter kinds.
* Queue extracted from Backend. We could use it elsewhere...
* Extracted an `EndpointDiscovery` strategy from `BalanceP2c` so that we
  can change that API if desired.
* Added a `Metadata` onto the backend -- the controller is going to have
  populate that.

This branch adds functions to the `linkerd-proxy-client-policy` crate
for converting the `linkerd2-proxy-api` outbound policy protobuf types
into the proxy's internal representations. These are not used in this
branch, but will be used in subsequent changes that actually wire up
outbound policy discovery.

This change was factored out from #2265, so that it can be reviewed
independently of the rest of the client policy discovery plumbing.

Note that this branch changes the `linkerd2-proxy-api` dependency to a
Git dep on the outbound policy branch (linkerd/linkerd2-proxy-api#165),
as the new APIs have not yet been published.