Method in the gateway to apply the validator changelog

[aakoshh]

This is a successor to #208

At the time that issue was implemented we were dealing with full Membership as the atomic change. Now we have consensus-shipyard/ipc-solidity-actors#225 which introduces the concept of a changelog.

This requires the creation of a method in Solidity that we call during the checkpoint transition to apply the validator change requests which were stashed since the last checkpoint.

The method should:

• Apply all changes to a data structure that is identical to one in the parent with the two priority queues
• Return the configuration number from the last change

If there are no changes, it should signal it to the caller so that it can skip trying to generate a diff to give to CometBFT, by:

• returning 0 for configuration number - any change would be a non-zero value, because 0 is for genesis, or
• return a (has_changed, configuration_number) tuple, or
• return the new active validator set, with an empty set for no change, or
• return a combo of a flag and a validator set...
• return the updates, ie. the diff itself, where an empty is a valid value; along with the new configuration number

The simplest I reckon is to return 0, and then Fendermint can run another query to get the active validators if the value is non-zero, and do the diff with CometBFT on its own.