FR: Expose GrpcForestoreRpc.close() via Firestore

[hiranya911]

GrpcForestoreRpc class has a close() method, which cleans up any allocated resources (client stubs, threads pools etc) and terminates the client. This should be exposed via Firestore interface so the users can gracefully terminate the client. For example:

Firestore fs;
try {
  fs = initMyFirestore();
  // Do stuff with fs
} finally {
  // On application shutdown or at any other suitable lifecycle handler
  fs.close();
}

[pongad]

Closed by #2807.