Redis external storage driver

[eamsden]

What was changed

Added a new external_storage_redis sample that implements a Redis-backed
Temporal external storage driver.

This includes:

• RedisStorageDriver plus a small RedisStorageDriverClient abstraction
• a redis.asyncio adapter in external_storage_redis/redis_asyncio.py
• sample workflow/activity definitions used by the integration tests

Why?

This adds a concrete Redis example for Temporal external storage so large
payload offloading can be exercised against a common backend with a
content-addressed key format.

Checklist

1. Closes (none)

2. How was this tested:

• uv run pytest tests/external_storage_redis/test_redis.py
• uv run pytest tests/external_storage_redis/test_redis_worker.py
• Unit suite result: 41 passed
• Worker suite result: 13 passed, 2 skipped
• The two skipped tests cover standalone activity execution, which some local
  Temporal dev-server builds currently report as disabled

3. Any docs updates needed?

• Updated the root sample list in README.md
• Added and updated external_storage_redis/README.md
• No temporal docs update needed