[ENH] registry module with lookup and tag register

[fkiraly]

This PR adds a registry module, with lookup and tag register.

Contents:

• a tag register in registry._tags. The current register from test_all_estimators is migrated there.
• a _lookup module with all_objects and all_tags lookup methods, as in sktime.
• populates object_type tag for all objects in skpro

In contrast to sktime, no base class registry is added, as this is now handled via the tag system and the object_type tag.