Mandatory dispatch class (#5515)

* Mandatory dispatch class

* Tweaks

* Docs

* Fix test

* Update frame/support/src/weights.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Introduce logic that was stated in PR.

* Use

* Docs.

* Fix test

* Fix merge

* Update frame/support/src/weights.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix.

* Fix

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>