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 backport of core#15936 [1/3]
https://github.com/bitcoin/bitcoin/pull/15936/commits/0e55bc6e7fe439404dc56093a0949395dae51e6b#