Add thread safety annotated wrapper for std::mutex

Summary:
Co-authored-by: Anthony Towns <aj@erisian.com.au>

This is part [1/5] of Core PR18635 : https://github.com/bitcoin/bitcoin/pull/18635/commits/79be4874209f71ba6428a80c40c9f028ac936c41

Test Plan:

ninja all check

Using clang. Check that the thread safety analysis is happy with it.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7877