feat: add KeyRangeCache for SpanFE bypass routing

This commit adds the KeyRangeCache that maintains a mapping from key
ranges to Spanner groups, splits, and tablets:

- Caches Range information (start/limit keys mapped to groups/splits)
- Caches Group information (tablets within groups, leader tablet info)
- Caches Tablet information (server addresses for each tablet)
- Manages server connections via ChannelFinderServerFactory

Key features:
- Efficient key range lookup using NavigableMap with ByteString keys
- Generation-based cache invalidation to handle schema changes
- Support for leader and local replica tablet selection
- Thread-safe operations with proper synchronization

The cache is populated from CacheUpdate responses received from the
server and provides routing hints for subsequent requests.

Depends on: ChannelFinder interfaces (PR #2)

This is part of the SpanFE bypass feature that enables location-aware
routing for improved latency.

Author: rahul2393