sidekick: automatically generate bidi functions

Some times have bidi functions, which will require Protobuf (see googleapis/google-cloud-swift#452) and gRPC.

- [ ] The generated client should contain a `GrpcClient` in addition to the `HTTPClient`.
- [ ] Both should share the same `auth` credentials.
- [ ] Both should use the same endpoint.
- [ ] The generator should emit methods for the bidi streaming RPCs.
- [ ] These functions should consume and return the sidekick-generated types.
- [ ] The functions should automatically convert to and from the Protobuf gencode types.
- [ ] For extra credit: filter out all the protos that are *not* used in the bidi RPCs. Some of the APIs that use these are huge and the bidi streaming RPCs need but a small fraction of all the code.