Partial implementation of supporting http routing annotations in generated grpc code

[devoncarew]

This is a partial implementation of support http routing annotations in generated grpc code. The main remaining thing is to augment package:grpc so that we can inspect the first request on a stream and use that to inform the http headers to use on that call. Unary requests are supported by this PR.

See #1018 for full context.

• add an additional proto (protos/google/api/annotations.proto)
• parse http annotations from the protos that we're generating (e.g., firestore)
• generate code to see if the values in a particular request match the http annotations; if so, apply the indicated http routing header for that call

For an example of the generated output from this PR see dart-lang/labs#285.

cc @mosuem