Consider adding validators package

Currently a lot of python tools just throw exceptions at us or even uses: `.is_valid()` and `.errors` properties. It is not type-safe at all. Sometimes it is hard to explain to people how to really use type-driven design and make invalid states unreachable.

My idea is that we need to write thin wrappers around popular typed validation packages: like `pydantic`, `cerebrus`, `schema`, etc to return really typed things.

We also need to think about API to be expressive and recognisable enough.

Related: https://gist.github.com/maksbotan/14b4bebda2acab98cdd158f85a970855
CC @maksbotan