[Feature]: Signers to collectively update request state in Emily

[djordon]

Feature - Signers to collectively update request state in Emily

1. Description

The signers should update Emily when a deposit request has been accepted, rejected, or completed. Right now only the coordinator updates Emily when it submits the bitcoin transaction sweeping the deposit, informing her that it has been accepted. But the signers should collectively sign requests to Emily so that it knows that the majority of signers agree that the deposit request has been accepted or rejected.

1.1 Context & Purpose

We want to ensure that failed deposit requests are noted as such in the bridge. We also want to make sure that no single signer has the power to incorrectly modify the state of a request in Emily.

2. Technical Details:

This task is quite involved, since it requires quite a few decisions.

1. What kind of message(s) do the signers send to Emily? How do that collectively sign the request? They could send multiple signatures, or perhaps a Schnorr signature using the latest aggregate key.
2. We'd need a new message types for signing requests to Emily.
3. We need new logic to coordinate Emily request signing rounds.
4. We need validation logic in the signers.
5. We need logic in Emily to verify the signature(s).

2.1 Acceptance Criteria:

• Emily only accepts requests from a signer when it knows that that a consensus majority of the signers agree with the contents.

3. Related Issues and Pull Requests (optional):