[stacks-signer] Verify messages coming from stackerdb event listener come from expected entities

[jferrant]

Audit: It might be worthwhile to add verification of all received messages that they come either from expected miner pub keys or registered signer pub keys. Stackerdb already does write verification, but to decrease reliance on this verification at the nodee, add message verification earlier. If we want to open the signer to allow accessing some external node later on, this could help prevent something like a man in the middle attack.