[ENH] make registry and lookup function public

[fkiraly]

This PR makes the registry module and the all_objects lookup function public. (previously, it was used only internally to retrieve all classes for testing)

It also adds some explanation on how to filter for optimizers and experiments.

A test for all_objects is included, to check against currently expected types in hyperactive.

The utility itself is more extnsively tested in skbase (from where the internals are interfaced).

[SimonBlanke]

Why is it important to make this public? How would this code be used in a practical example?

[fkiraly]

Why is it important to make this public? How would this code be used in a practical example?

not that important, but:

• users like this feature, it allows to filter programmatically and in python - "show me all optimizers that search locally" etc
• another package can ask this package, "what optimizers do you have"