BeanValidation Directives

[t1]

Expose the BeanValidation annotations as GraphQL custom directives, e.g.:

class Person {
    @Email String email;
}

input Person {
    email: String @email
}

As the BeanValidation annotations are not under our control, we could use mixins (#316).

In addition to being documentation for client developers, e.g., a web app may use the directives to render the correct input type and validate it while the user is typing. I don't know if something like this is already being used in existing frontend frameworks.