Table of Named Restore Points

With the introduction of `citus_create_restore_point` function, we now be able to safely capture distributed and consistent restore points. However, it is difficult to keep track of existing restore points and the state of the cluster during the restore point.

Keeping track of it can be done by the client but it would bring extra burden to a Citus customer. Putting this feature to Citus would allow a broader audience to benefit from it.