feat(gengapic): regapic server-streaming

[noahdietz]

This adds support for REGAPIC Server Stream RPCs. Since server-stream RPCs in GAPICs must return the gRPC-defined, RPC-specific "stream client" interface, we must implement that interface around the HTTP-JSON stream (the Response Body) opened by the HTTP-JSON request. An RPC-specific implementation of that stream client interface is generated in the GAPIC that utilizes the gax.ProtoJSONStream helper to decode the stream contents into the proto messages. The behavior of this generated stream client is the same as documented by the gRPC-defined interface.

When the stream is done/closed, io.EOF is returned.

If the context.Context used to initialize the stream is canceled, the invocations of Recv fail.

When the stream is finished or in the event of an error, the gax.ProtoJSONStream is closed by the generated stream client.

[noahdietz]

I regenerated the Firestore client with these changes and with REST transport generation enabled. I was then able to successfully consume a RunQuery REST stream.