check that wsts messages are signed by the specified signer_id

[xoloki]

Description

sBTC messages contain a signature and a public key, in addition to the payload. Verifying the message shows that it has not been tampered with.

WSTS message payloads either contain a signer_id, or are coordinator messages. So in addition to verifying the signature, we also need to show that the public key used to sign was the correct public key for that signer (or the coordinator).

Closes: #578

Changes

Call verify on the sBTC message, then check that the message was signed by the correct public key.

Testing Information

All unit and integration tests still pass.

Checklist:

• [ x] I have performed a self-review of my code
• [ x] My changes generate no new warnings
• [ x] New and existing unit tests pass locally with my changes
• [ x] Any dependent changes have been merged and published in downstream modules

[djordon]

Since the other one is up to date, might as well close this one.