util: Add ReadSettings and WriteSettings functions

Summary:
Currently unused, but includes tests.

Backport of PR15935 part [1/2] : https://github.com/bitcoin/bitcoin/pull/15935/commits/eb682c5700e7a9176d0104d470b83ff9aa3589e8

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8011