Add memcached store implementation

[Copilot]

This PR adds a complete MemcachedStore implementation to the KV Store Adapter, providing high-performance distributed caching capabilities through the pymemcache client.

Implementation Details

The MemcachedStore extends BaseManagedKVStore and provides full async support with the following key features:

• Native TTL Support: Leverages memcached's built-in expiration mechanism
• Key Length Handling: Automatically hashes keys exceeding memcached's 250-character limit using MD5
• Flexible Initialization: Supports both direct connection parameters and existing pymemcache client instances
• Robust Error Handling: Proper connection testing and StoreConnectionError integration

Usage Examples

from kv_store_adapter.stores.memcached import MemcachedStore

# Connection with host and port
store = MemcachedStore(host="localhost", port=11211)

# Using existing pymemcache client
from pymemcache.client.base import Client
client = Client(("localhost", 11211))
store = MemcachedStore(client=client)

# Standard KV operations with TTL support
await store.put("collection", "key", {"data": "value"}, ttl=3600)
result = await store.get("collection", "key")

Memcached Limitations

Due to memcached's architecture, certain operations have inherent limitations that are transparently handled:

• keys(), clear_collection(), and list_collections() return empty results since memcached doesn't support key enumeration
• Keys longer than 250 characters are automatically hashed to ensure compatibility
• These limitations are clearly documented in both code and user documentation

Dependencies and Integration

• Added pymemcache>=4.0.0 as an optional dependency in pyproject.toml
• Updated documentation in both README.md and DEVELOPING.md
• Comprehensive test suite with proper CI integration using @pytest.mark.skip_on_ci
• All code passes linting with proper type safety and style compliance

Testing

The implementation includes extensive tests covering:

• Basic store operations and initialization patterns
• Long key handling and automatic hashing
• TTL functionality and expiration behavior
• Connection error handling and recovery
• Memcached-specific limitations and edge cases

This adds memcached as a production-ready store option alongside Redis, Elasticsearch, and other existing implementations, providing users with another high-performance caching solution for distributed environments.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add memcached store</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #6

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