support http and routing annotations

We should support http and routing annotations in our grpc service generator.

The best docs for this is at:

- https://google.aip.dev/client-libraries/4222

with additional documentation at:

- https://github.com/googleapis/googleapis/blob/master/google/api/http.proto
- https://github.com/googleapis/googleapis/blob/master/google/api/routing.proto

Our grpc service generator should look for http and routing annotations and add the correct http headers for service calls (`'x-goog-request-params'`).

There's an example of passing the headers manually at https://github.com/dart-lang/labs/pull/240/files#diff-64691e9691a43f3ccbf3f98916838891c0a179c135a6bea02a79e0f05a51665fR160.

cc @craiglabenz, @mosuem

Start with the gRPC service generator and read the linked AIP-4222, http.proto, and routing.proto documentation, along with the referenced Dart example. Trace how generated service calls are assembled. Done means supported HTTP and routing annotations produce the correct x-goog-request-params headers.

Written by the indexing model from the issue text.