feat: add memcache distributed cache support

[petercinibulk]

What changed

Adds Memcached L2 cache support through a new MemcachedDistributedCache provider backed by aiomcache.

Details

• Adds the cache_sync.providers.memcache provider package and lazy public export.
• Adds a memcache optional extra and includes it in the all provider set.
• Serializes values with the existing serializer interface, defaulting to PickleSerializer like Redis.
• Converts cache keys to Memcached-safe bytes and hashes keys or prefixes that exceed Memcached key constraints.
• Documents installation, public API, provider capabilities, and README usage.

Validation

• uv run ruff format
• uv run ruff check
• uv run ty check
• uv run --group dev python -m pytest