[stacks-signer] Retrieve a proposed block from the .miners stacker db instance

[jferrant]

Description

Update stacks signer to retrieve proposed blocks from miners

Context

Miners should be writing proposed blocks to the .miners contract for signers to observe and validate and sign and return back to the miner to append to the block.

Expected Approach

Add the .miners contract to the list of observed stacker db contracts so that the event listener receives these serialized proposed blocks and deserialize them into a Nakamoto Block. Verify that the block received was written by the current miner (this will require a call to a stacks node endpoint to retrieve the current miner slot id. Must do this to prevent signing blocks that come from random miners who are not actually one who one the tenure)

Acceptance Criteria

If a block is written to the .miner contract, ensure that the signers receive this block and are able to deserialize it