Feature - Signers create bitcoin sighashes for bitcoin sign requests

1. Description

This is ticket corresponds to point (4) in 3.1 in #782. Signers generate sighash digests, from the incoming sign request.

1.1 Context & Purpose

Each signer generates the bitcoin transaction and associated sighashes from the sign request.

2. Technical Details:

Each signer needs to load the current bitcoin state and use that to generate an UnsignedTransaction. Once they have that they can easily create sighashes.

2.1 Acceptance Criteria:

• Signers fetch the BTC state from the database and construct a transaction and sighash.

3. Related Issues and Pull Requests (optional):

Relates to:

• [Feature]: Signers check sighash before participating in signing rounds #793
• [Feature]: Validate bitcoin transaction sign requests #286
• [Design]: Signing bitcoin transactions #782
• [Feature]: Signers store bitcoin sighashes and decisions #794