feat(bigtable): add ClientConfigurationManager (#19986)

## Summary

- Add `ClientConfigurationManager` in `bigtable/internal/transport`:
polls `GetClientConfiguration` on a fixed interval, parses the response
into a typed `clientConfig`, and fans changes out to registered
listeners. Supports RPC retry, validity-window fallback, and `Close()`
shutdown that waits for in-flight polls.
- Test suite covering construction, polling, listener fan-out,
transient-error retry, and `Close` semantics.

Author: sushanb