Add validators to forms

[sasetz]

The validators are need for each form request, so we need them. @IlliaPonomarov please provide a list of some sort of what methods need to have them now

[IlliaPonomarov]

Write validator which checks that user exists by email or username. Example of validator you can find here https://github.com/IlliaPonomarov/AvoChad/blob/main/avochad-backend/src/main/java/com/example/avochadbackend/utility/validators/UserValidator.java