[stacks-core] Update the stacker-db contract from the list of registered signers

There should be a map of signer keys (public keys) to stacked stx in pox-4. Utilize this map to update the stacker-db config of the stacker-db instance to enable these signers to write to the appropriate slots. Note that we will need to sets of stacker-db slots. One for signers for cycle N, one for signers of cycle N+1.