Is your feature request related to a problem? Please describe.

• use of "capabilities" rather than "tags" in classification module
• unclear naming of _all_tags method
• inconsistent tags for composition algorithms (see Refactoring Stacking, Multiplexer, Ensembler and TransformedTarget Forecasters #977)
• integration in all_estimators function to make them usable in finding/filtering models

Describe the solution you'd like

• refactor capabilities into tags
• rename _all_tags to get_tags and make public
• add tag kwarg to all_estimators
• add tests to catch inconsistent tags