Method in the Gateway to return the checkpoint interval

[aakoshh]

The Fendermint interpreter needs to know the checkpointing boundary to know when to construct a checkpoint.

An alternative to this is if the Gateway has an "end block" method and does the height checks on its own, but then it needs to return much more than if we break these out into methods, and Fendermint would tell from the return value whether a transition has taken place or not.

[dnkolegov]

bottomUpCheckPeriod method of GatewayGetterFacet can be used for that