Access Service from validator class

[sandstrom]

It would be neat if one could access other ember services from within a validator class.

I understand the idea of using POJO and vanilla classes (validated-changeset is generic).

But since this is (in practice) very tied to Ember, in my view it'll make more sense to be coupled with Ember more tightly.

Partly related: https://github.com/poteto/ember-changeset-validations/discussions/306