support http and routing annotations

[devoncarew]

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

[devoncarew]

Somewhat related, a PR to add support for reading default_host and oauth_scopes options: #991.