Add configurable encoder classes to standalone clients

[lh0156]

What

Adds support for configuring a custom Encoder subclass on standalone sync and asyncio clients and their connection pools.

Closes #3181

Why

The default encoder rejects application-specific Python value types. Users can now implement custom serialization/deserialization while retaining redis-py's existing encoding configuration.

Design

• Adds encoder_class to sync and asyncio Redis constructors.
• Propagates the class to the connection pool and connection instances.
• Instantiates an encoder per connection/helper instead of sharing mutable encoder instances.
• Exposes Encoder from redis and redis.asyncio.
• Preserves the default Encoder behavior when no custom class is supplied.
• Documents the connection-isolation contract.
• Limits this change to standalone clients and pools; Cluster/Sentinel behavior is unchanged.

Tests

• Added sync and asyncio regression coverage for:
  • custom encoding and decoding
  • pool-created connection encoders
  • pool helper encoders being distinct instances
  • client-to-pool configuration propagation
• 157 passed, 4 skipped for tests/test_connection.py tests/test_asyncio/test_connection.py
• 17 passed for tests/test_client.py tests/test_asyncio/test_client.py
• Ruff check/format, compileall, and git diff --check pass.

This is a draft while CI and maintainer feedback run.