[stacks-signer] Retrieve the list of registered signers from the pox-4 contract for a given reward cycle

[jferrant]

Description

Retrieve signer info from the stacks node instead of the config file.

Context

Currently stacks-signer operates by using a config file to store the list of signers public keys, their IDs, and their reward slot numbers. All this data should be retrieved from the node. This depends on the clarity workstream pox-4 function existing.

Expected Approach

This will require a read only contract call to the stacks node (Most likely it will be stored inside pox-4) OR require a call to some stacks node endpoint that reads the sortition db.

Acceptance Criteria

The signers can spin up with no data in the config file concerning the other signers in the network and should be able ot retrieve the list of current signers, their public keys, and their assigned reward slots.