settings: Add update/getPersistent/isIgnored methods

Summary:
Add interfaces::Node methods to give GUI finer grained control over
settings.json file. Update method is used to write settings to the file,
getPersistent and isIgnored methods are used to find out about settings
file and command line option interactions.

This is a partial backport of core#15936
https://github.com/bitcoin/bitcoin/pull/15936/commits/0e55bc6e7fe439404dc56093a0949395dae51e6b

Depends on D15064

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15065