This commit adds the server abstraction interfaces for location-aware
routing:

- ChannelFinderServer: Interface representing a Spanner server endpoint
  with address, health check, and channel access
- ChannelFinderServerFactory: Factory interface for creating and caching
  server connections
- GrpcChannelFinderServerFactory: gRPC implementation that creates and
  manages gRPC channels for different server endpoints

These interfaces enable the client to maintain connections to multiple
Spanner servers and route requests directly to the appropriate server
based on key location information.

This is part of the experimental location-aware routing for improved
latency.