[moetemp]

What was changed

This PR adds StreamBackend, the base class of the Stream backends, and stream_backends= on the client, the Replayer and WorkflowEnvironment. Each Stream ref keeps its backend's name, so any process can route a ref it gets.

• A Worker uses its client's backends, and applies them after its plugins, so a plugin that replaces the Workflow runner doesn't drop them.
• With more than one backend, exactly one must have default=True, since a replayed create must use the same backend.
• The generated Stream service client and the hand-written writer client start their operations on the crate's runtime. Their create_nexus_client gives a Nexus client that converts each model with the client's converters and codec, as Workflow code does, and calls the runtime in place of an endpoint.
• The Nexus client runs the codec once for each request and once for each response, on all of their payloads, since a remote codec costs a round trip for each call.

Why?

The Redis and memory backends and the Workflow, Activity and client calls all get their runtime from this registration.

Open questions.

• StreamBackend subclasses SimplePlugin for its hooks, but plugins= refuses it. Is that OK?
• The default backend is native, though this SDK has no native backend yet. Is that the right default?
• ClientConfig.stream_backends is Required, so a hand-built ClientConfig must now set it. Is that OK?

Part of the Streams review series.

Checklist

1. How was this tested:
   • poe lint and pytest tests/contrib/streams.

[moetemp]

Closed in favor of #381, the combined branch, which has this change.