feat: expose shared session APIs

[Jay-ju]

Summary

• expose an opaque LanceSession with configurable index and metadata cache limits
• allow datasets to share a session through lance_dataset_open_with_session while preserving the existing open API
• expose cache hit, miss, entry, and retained-byte statistics
• add C++ RAII wrappers, documentation, and lifecycle/cache reuse coverage

[lance-gatekeeper]

⚠️ Gate recommendation: approve with a non-blocking risk.

The explicit session handle matches the upstream Lance ownership model, preserves the existing open APIs, and correctly demonstrates metadata-cache reuse and lifetime independence across Rust, C, and C++.

A non-blocking risk remains in the pinned upstream implementation: cache namespaces include the dataset URI but not object-store options. If one session opens the same URI against different physical endpoints, separate sessions are the safe mitigation until store identity is incorporated into the upstream cache key.