Stacker Signer Reference Implementation

[kantai]

This is the task of creating a stacker signer. The implementation would run as a separate process.

This task will probably need to be broken down further, a lot of the work is going to be in cribbing the sBTC signer implementation and merging it with an API for querying unsigned blocks, but there may be other tasks. Once the better blocks implementation is closer to unblocking this step, the tasks will probably be more clear.

[jcnelson]

Re-assigning this based on current peoples' responsibilities.