[Cashtab] [Deprecate BCH msg signing p2/3] Implement local msg signing

Summary:
T2730

Depends on D12626

Implement message signing without use of the BCH param. Compare new method to legacy method.

Test Plan: npm start and sign some msgs. Look for The signatures match in the dev console.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12627