Add async support to Speech API

[daspecster]

Add async support to Google Cloud Speech API.

[daspecster]

@dhermes @tseaver

The Operation response for Speech async requests seems pretty generic and possibly similar to google.cloud.operation.Operation.

Should I just add a non-protobuf response parser toOperation? (instead of from_pb() a from_api_repr).

[daspecster]

The more I look at it, they seem very different. I'm going to just make speech.operation unless you think I should put it in google.cloud.operation.

[dhermes]

Check out the proto for the service. It's the very same Operation used by Bigtable and others.

[daspecster]

Speech sync recognize is implemented via JSON/REST at the moment.

Should I just implement asyncrecognize with gapic GAX? This seems weird to me since, grpc is an optional requirement. (Was my usage of gapic and grpc correct in that wording?)

Also it appears that gapic isn't available for speech yet. GAX and GRPC are though.

[daspecster]

Done with #2663(GAX) and #2426(JSON/REST).