Lists the names of the snapshots on this topic. Snapshots are used in
[Seek](https://cloud.google.com/pubsub/docs/replay-overview) operations, which allow you to
manage message acknowledgments in bulk. That is, you can set the acknowledgment state of messages
in an existing subscription to the state captured by a snapshot.
Create a request for the method "snapshots.list".
This request holds the parameters needed by the pubsub server. After setting any optional
parameters, call the
AbstractGoogleClientRequest.execute() method to invoke the remote operation.