Check that Schnorr signatures enforce domain separation

[AaronFeickert]

A recent PR showed an instance where a Schnorr signature is used without domain separation. This is a good opportunity to check other uses of such signatures and ensure they use domain separation.

The underlying tari-crypto implementation of Schnorr signatures provides for optional domain separation. If a domain separator is not supplied, a default is used.

This is risky. Signatures intended for use in different contexts must use unique domain separation; otherwise, it may be possible to replay a signature in an unexpected context, which can have very bad consequences. The intent of making it optional was simplicity, but it also introduces real risk if not carefully applied.

This issue should be closed when it's confirmed that domain separation is used in all cases throughout the codebase.

Separately, I think it's a good idea to make the underlying tari-crypto library enforce a domain separator, but that's a PR for another day.