Introduce validation method and class in XBlock API

@cpennington and @dmitchell please review

You can see the Studio extension of the Validation class in common/lib/xmodule/xmodule/validation.py in https://github.com/edx/edx-platform/pull/5710 (which is still WIP so is not yet commented or tested).
Yeah, we haven't gone back and converted anything yet, but I've been trying to get new documentation into the napoleon format, when I catch it.