feat: signer monitoring server

[hstove]

This is the same code as https://github.com/stacks-network/stacks-core/pull/4719, but cherry-picked against master.

Note that there will be some (not too bad) conflicts when merging master into develop, for two reasons:

• There were other signer config fields added to develop before this PR, so things like build_signer_config_tomls will have a conflict
• A few stacks_client.rs functions were either renamed or added in develop

[wileyj]

what about PR'ing the expected conflicts here to reduce the friction? are they things that shouldn't be included in a point release?