[ENH] Add `ReprMixin` to share `__repr__` formatting across OpenML classes

[JATAYU000]

Add a ReprMixin class (in openml/utils.py) that provides:

• __repr__ which calls self._get_repr_body_fields()
• _apply_repr_template to produce the formatted string

Refer Discussion
#1567

[JATAYU000]

I will be working on it.