Streaming client prototype: EngineClient interface and stream handler

[verult]

A prototype of EngineClient.create_job_and_get_result() covering the happy path only; it doesn't handle stream disconnects, so it never makes a GetResult call over the stream.

The prototype contains:

• A long running stream handler to receive stream responses
  • Runs in the asyncio context.
  • Handler restart logic (including making a GetResult request for every outstanding job in Response Listener) is missing
• Response listener
• EngineClient.create_job_and_get_result() implementation to send requests to the request stream directly and receive responses from the Response Listener.
  • The request stream data structure is not yet thread-safe.

cc @maffoo @wcourtney

[verult]

The actual implementation has been merged (#6199). Closing