Update withdrawal signature design to be more wallet friendly

[netrome]

The current withdrawal payloads requires arbitrary messages to be signed. These signatures are impossible to create through the current signing API in Hiro wallet. This is by design, since the wallet does not want to support someone accidentally signing a bitcoin or stacks transaction through that API. For the same reason, we should not build a solution that allows an application to maliciously trick users into signing sBTC transactions.

For the first alpha prototype, we had to do an ugly workaround to make the alpha coordinator able to parse signatures created through the wallet. See this comment and PR: https://github.com/Trust-Machines/stacks-sbtc/pull/602#issuecomment-1649523475.

Going forward, we should have a dialogue with the wallet team about which signature format they could support and how they could implement a dedicated function to produce a sBTC withdrawal message signature. This is likely going to entail a wire format update, which should be reflected in this documentation.

Definition of done

The signature in the withdrawal payload has a format that is secure and easy to support in wallets.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.