[electrum] use libsecp256k1 for pubkey recovery (from sig and msg)

Summary:
Partial backport of electrum#5947
https://github.com/spesmilo/electrum/pull/5947/commits/288d793893e65466ae60bf8b60963b58b849d48e

Depends on D16690

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16705