investigate if proto-plus to protobuf response conversion is needed for a streaming response.

[ohmayr]

For a proto plus method, we do the following type conversion for a non-streaming response:

pb_resp = {{method.output.ident}}.pb(resp)

We need to investigate if this is needed for a streaming response for both sync and async REST.

Related PR: https://github.com/googleapis/gapic-generator-python/pull/1428