[backport#14047 1/4] CKey: add method to negate the key

Summary:
https://github.com/bitcoin/bitcoin/pull/14047/commits/463921bb649d644f79f9d7f0f96f10aa0d165f76

Partial backport of Core PR14047

Test Plan:

ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7393