MV Shiva

**Note**: The Google Auth Library have built-in mechanisms to handle clock skew in ComputeEngineCredentials.java. `GoogleCredentials` manages token refreshing, validity and parsing of the JWT token internally.

Envoy proto sync to latest versions in grpc-java is blocked until `sourced_metadata` is implemented. Implementing sourced_metadata from [rbac.proto](https://github.com/envoyproxy/envoy/blob/a0c96b389d2ef44ff207bb17678a5c5eabdbbadb/api/envoy/config/rbac/v3/rbac.proto#L311) would be cross-language, so would require a gRFC (yet to be published).

**Summary of Changes** This pull request refactors the grpclb load balancer's PICK_FIRST mode to delegate its logic to a standard pick_first load balancing policy. The key changes are as follows:...

Fixes #12395