[moetemp]

What was changed

This PR adds the Redis Stream backend (temporalio.contrib.stream.RedisStreamBackend) and the in-memory backend for tests (temporalio.contrib.stream.testing.MemoryStreamBackend).

• A client creates a Stream on either with Client.create_stream, which takes backend= for a backend that isn't the default. The package has no create of its own.
• Both call the temporalio-streams runtime that B1 links. The memory backend has Redis's rules, so tests can swap the Store with no other change.
• The clients of one temporalio.runtime.Runtime share one Redis runtime object and its connections. So a service that makes a client for each tenant opens no more connections.
• CI installs redis-server on Linux and macOS, where the Redis lane fails instead of skipping. Redis has no official Windows build, so the lane skips there.
• The live tests run on both backends. They start their own redis-server, or use TEMPORAL_STREAMS_REDIS_URL.
• The Activity part of _extension.py and the Worker and Replayer config are in S7, with their first users.

Depends on S5 (#375), and on G3 (#369) and B1 (#370) through feature-streams. No changelog fragment, since S9 has the Streams entry.
Part of the Brook review series.

Why?

These are the first backends that run. From here on, the tests run live on both Stores.

Open question. ExtensionStreamBackend is a public class in a private module, as the base of both backends. Is that OK?

Checklist

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