auth: consider sharing credential in gRPC pool

Today any time a pool of connections are created we create a credential object for each connection dialed.

Consider the case where a user specifies a pool of 10 and is using ADC compute credentials. Currently every hour when a credential expires all 10 credential objects need to re-auth to the metadata service. If we instead share just the credential object every hour only 1 re-auth call would be made. O(n) -> O(1).