Add TTL-based snapshot caching for DuckLake catalog

[shefeek-jinnah]

Summary

Adds configurable time-to-live (TTL) based caching for snapshot resolution in DuckLakeCatalog, allowing users to balance data freshness against query performance.

Changes

• Added SnapshotConfig struct with ttl_seconds: Option<u64> field
• Added DuckLakeCatalog::new_with_config() for custom snapshot configuration
• Implemented three caching modes:
  • TTL = 0 (default): Always query latest snapshot
  • TTL > 0: Cache snapshot for specified duration
  • TTL = None: Cache snapshot forever
• Thread-safe implementation using RwLock with double-check locking

API

// Default: always fresh (TTL=0)
let catalog = DuckLakeCatalog::new(provider)?;

// Custom TTL: cache for 5 seconds
let config = SnapshotConfig { ttl_seconds: Some(5) };
let catalog = DuckLakeCatalog::new_with_config(provider, config)?;

// Cache forever
let config = SnapshotConfig { ttl_seconds: None };
let catalog = DuckLakeCatalog::new_with_config(provider, config)?;

Testing

- 5 new unit tests covering all TTL modes and concurrent access
- All existing tests pass (82 total)

[zfarrell]

Noting the logic bug we discussed on the call: it's currently possible in all caching configurations other than none for a snapshot to change during a user's query and to have the user query be served from multiple snapshots. We should address this so that only one snapshot id is used for the user's query.

[zfarrell]

@shefeek-crypto - should we go ahead and close this (in favor of #6), or is there anything here that you're hoping to keep?