[Feature]: Make sure signers go through the validation code path

[djordon]

Feature - Make sure signers go through the validation code path

1. Description

The code that signs bitcoin transactions does not go through the bitcoin validation code path. It needs to.

1.1 Context & Purpose

A signer needs to validate a bitcoin sign requests before it provides a signature. Right now the signers always sign the request partly because they don't try to validate it.

2. Technical Details:

2.1 Acceptance Criteria:

• We call the function that is supposed to do validation of the bitcoin transaction before providing signature shares.

3. Related Issues and Pull Requests (optional):

This is the last part of #286.