#6386 Add Priority system for validators
#6496 #6386 - validator chain should reject invalid callbacks
#6496 #6386 - minor cleanups, simplifying checks around `$callback` validity
#6496 #6386 - adding `@group` annotations for newly introduced tests
#6496 #6386 - inlining `$this->attach()` and `$this->plugin()` calls
#6496 #6386 - reverting minor bc-break (`attach` method should keep a compatible signature)
#6496 #6386 - removing useless test that checks against an invalid type-hint usage
#6496 #6386 - reverting unrelated test changes
Merge branch 'feature/#6386-#6496-support-validator-priority' into develop

Close #6386
Close #6496