[stacks-signer] sign a stx transaction

[jferrant]

Description

Signers must broadcast contract call stx transactions. They must be signed by 70%+ of signers to be valid.

Context

Example stx transactions: no quorum reached, transaction rejection (quorum reached but vote was no)

Expected Approach

Take a generic stx transaction and sign its contents using the signing round across stacker db.

Acceptance Criteria

• [ ] PR adds the feature ...