feat: Support delayed settings_source assignment for built-in settings objects

[mkundu1]

The following 2 syntaxes are now supported:
1.

x = MyBuiltinSettings(settings_source=<solver_session>/<settings_root>, ...)
assert isinstance(x, <conventional settings class>)

x = MyBuiltinSettings(...)
assert isinstance(x, MyBuiltinSettings)
x.settings_source = <solver_session>/<settings_root>
assert isinstance(x, <conventional settings class>)
x.settings_source = <solver_session>/<settings_root>  # RuntimeError thrown from <conventional settings class>, re-assignment is not allowed. (Ideally, it should be AttributeError, need to fix from flobject.py.)