[Feature]: Signers check sighash before participating in signing rounds

[djordon]

Feature - Signers check sighash before participating in signing rounds

1. Description

This is ticket corresponds to point (7) in 3.1 in #782. Signers need to look up the sighash/digest from the database and decide whether to participate in the signing round based off of the stored decision.

1.1 Context & Purpose

One of the last steps of the bitcoin validation flow is refusing to sign sighashes that a signer cannot or will not sign for. This decision is stored before hand, we just need to use this decision when the coordinator asks for a signature.

2. Technical Details:

When a signer receives a sign requests message from the coordinator, the signer looks up their decision from the database and participates if the decision was to participate.

2.1 Acceptance Criteria:

• Signers do not participate in signing rounds where they have decided not to sign a particular sighash.

3. Related Issues and Pull Requests (optional):

Relates to

• [Feature]: Validate bitcoin transaction sign requests #286,
• [Feature]: Make sure signers go through the validation code path #696,
• [Design]: Signing bitcoin transactions #782