feat(cache): add cache store abstraction layer

Implement reusable cache store abstraction with in-memory and Redis
backends as foundation for prompt caching feature (PR1 of progressive delivery).

- Add CacheStore protocol defining cache interface
- Implement MemoryCacheStore with LRU, LFU, and TTL-only eviction policies
- Implement RedisCacheStore with connection pooling and retry logic
- Add CircuitBreaker for cache backend failure protection
- Include comprehensive unit tests (55 tests, >80% coverage)
- Add dependencies: cachetools>=5.5.0, redis>=5.2.0

This abstraction enables flexible caching implementations for the prompt
caching middleware without coupling to specific storage backends.

Signed-by: William Caban <willliam.caban@gmail.com>

tests/unit/providers/utils/cache/__init__.py

@@ -0,0 +1,7 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""Unit tests for cache store implementations."""