[schnorr] Refactor the signature process in reusable component

Summary: This will make it easier to implement more evolved schnorr constructs such as blinded signatures and multisig.

Test Plan: Adapted test cases to reflect the changes.

Reviewers: #bitcoin_abc, markblundeberg, Fabien

Reviewed By: #bitcoin_abc, markblundeberg, Fabien

Subscribers: Fabien, markblundeberg, jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2457