[stacks-signer] Enable signing multiple messages in a single signing round

[jferrant]

Description

Enable parallization of message signing by passing a vector of messages to sign through stacker-db instead of a single message.

Context

This is required to enable construction of consolidation transactions.

Expected Approach

We should preferably not have to mess with a signing_round state machine.
Update stacker-db messages to include a vector of things to sign in a signing round rather than a single message.
Update the logic of the signing round to iterate over these things and return outputs that correspond to each input.

Acceptance Criteria

• PR adds the feature