Fix regression: make BASE_ONLY_SECTIONS no longer a frozen set (#16685)
Fixes #16684
Summary of the issue:
In #16636 / d7facd1, config.ConfigManager.BASE_ONLY_SECTIONS underwent a mutation from a set to a frozenset.
Thhis modification hinders extensions from easily expanding this set.
There are perfectly legit scenarios where an extension’s settings should not be overridden by profiles. Take webAccess, for instance: accessolutions/WebAccessForNVDA@4251aae/addon/globalPlugins/webAccess/config.py