Method in Checkpoint actor to add new unsigned checkpoint

[aakoshh]

As a Fendermint interpreter, at the end of the checkpoint period, I will construct a BottomUpCheckpoint deterministically from the ledger, and I want to store this checkpoint in the Checkpoint actor created in consensus-shipyard/ipc-monorepo#207 , so that everyone can see what it is, and we have a place to accumulate signatures. To be able to check signatures the actor will also need to remember the validator set at the time the checkpoint was added.

[aakoshh]

I think the checkpoint should look as follows:

struct BottomUpCheckpoint {
  // Child subnet ID, for replay protection from other subnets where the exact same validators operate.
  // Alternatively it can be appended to the hash before signing, similar to how we use the chain ID.
  subnet_id: SubnetID, 

  // The height of the child subnet at which this checkpoint was cut.
  // Has to follow the previous checkpoint by bottom_up_checkpoint_period 
  block_height: u64, 

  // The ID of the validator set which is going to sign the next checkpoint.
  // This one expected to be signed by the ID reported in the previous checkpoint.
  // 0 could mean "no change".
  next_configuration_number: u64, 

  // Some kind of hash over the bottom-up messages.
  // By not including cross messages here directly, we can be compatible with IPLD Resolver based 
  // approach where the messages are fetched with Bitswap and provided by Fendermint, or the full-fat
  // approach we need with Lotus, where the messages are part of the relayed transaction.
  cross_messages_hash: bytes32
}

The method signature to add such a checkpoint should look like this:

createCheckpoint(
  checkpoint: Checkpoint,   
  // Root hash of the Merkle tree built from the validator public keys and their weight.
  validator_set_root_hash: bytes32, 
  totalWeight: uint256
)  external systemActorOnly 

[dnkolegov]

Implemented https://github.com/consensus-shipyard/ipc-solidity-actors/pull/202