A connection to Cloud Pub/Sub for topic-related administrative operations.
This interface defines pure-virtual functions for each of the user-facing overload sets in TopicAdminClient. That is, all of TopicAdminClient's overloads will forward to the one pure-virtual function declared in this interface. This allows users to inject custom behavior (e.g., with a Google Mock object) in a TopicAdminClient object for use in their own tests.
To create a concrete instance that connects you to the real Cloud Pub/Sub service, see MakeTopicAdminConnection().
The *Params nested classes
Applications may define classes derived from TopicAdminConnection, for example, because they want to mock the class. To avoid breaking all such derived classes when we change the number or type of the arguments to the member functions we define lightweight structures to pass the arguments.
