Inconsistency in credentials specification for CPS Publisher and Subscriber #1959

Both the Publisher and the Subscriber class state "If no credentials are provided, the [Publisher|Subscriber] will use application default creentials through GoogleCredentials#getApplicationDefault. However, while Subscriber.Builder has a setCredentials method, Publisher.Builder does not. Moreover, from what I can see, Subscriber does not actually use the credentials set via Subscriber.Builder.setCredentials. If we are going to allow one to set credentials this way, we should be consistent between the Publisher and Subscriber and ensure we use the provided credentials. Otherwise, we should remove the method from the Subscriber.Builder.