Create a rollup keeper interface for use when mocking unit tests

[natebeauregard]

Currently, the rollup keeper is passed around and referenced as a struct with private fields. This makes it difficult to mock the x/rollup keeper behavior in unit tests when trying to test information from

We should add a new interface (e.g. RollupKeeper, KeeperI) that includes the expected keeper functions that we expect to mock. We could also pass this interface around instead of the x/rollup keeper struct when calling the keeper functions outside of the x/rollup module.

Lastly, unit tests should be added for the monomer ante handlers and the l1_data_decorator that mock the rollup keeper's GetL1BlockInfo func.