[moetemp]

What was changed

This PR adds StreamBackend, the base class of the Stream backends, and stream_backends= on the client and the Replayer. Each Stream handle stores its backend's name, so any process can route a handle 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 select the same backend.
• The generated Stream service and writer clients 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.

Depends on S3 (#373), and on B1 (#370) through feature-streams. No changelog fragment, since S9 has the Streams entry.
Part of the Brook review series.

Why?

The S6 backends and the Workflow, Activity and client handles all get their calls from this registration.

Open questions.

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

Checklist

1. How was this tested:
   • poe lint, poe gen-docs and pytest tests/contrib/streams. Fork CI passes, except the arm jobs, since the fork has no arm runners.