bugfix: Add ABC to classes using @abstractmethod

[MrCrystAll]

Bug/Problem

The config objects (ObsBuilder, ActionParser, ...) aren't inheriting ABC despite using the @AbstractMethod decorator.
From the python documentation:

Using this decorator requires that the class’s metaclass is [ABCMeta](https://docs.python.org/3.14/library/abc.html#abc.ABCMeta) or is derived from it. A class that has a metaclass derived from ABCMeta cannot be instantiated unless all of its abstract methods and properties are overridden.

ABCMeta can either be used by writing metaclass=ABCMeta or by inheriting ABC (see https://docs.python.org/3.14/library/abc.html#abc.ABC).

The reason why i'm making a fix for it isn't only regarding the documentation, inheriting ABC allows type introspection scripts to mark the object as abstract. I also believe some IDEs base their override menu on the ABC inheritance