Mandatory dispatch class (#5515)

* Mandatory dispatch class

* Tweaks

* Docs

* Fix test

* Update frame/support/src/weights.rs

Co-Authored-By: joe petrowski <[email protected]>

* Introduce logic that was stated in PR.

* Use

* Docs.

* Fix test

* Fix merge

* Update frame/support/src/weights.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Fix.

* Fix

Co-authored-by: joe petrowski <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>