refactor(launcher): add RunningStep interface

[miltolstoy]

Refactoring according to this comment:

Instead of returning tuple, I suggest to perform some refactoring:

1. Declare interface for running step explicitly, as an abstract base class

2. Use type declaration of this interface for type checking with mypy

3. Include error status into this interface instead of returning it from the "start" and "finalize" functions separately

4. Pass the object into active_background_steps instead of passing separate methods