Added validators for users and recipes (#1052)

* Added validators for users and recipes

provide a simple get api, allowing to test for existence of
- user by username
- recipe by slug
- group by name (not tested yet)

* updated formatting

* Use group_id+slug for recipes, use ValidationRespone

* Fixed Flake8 errors and warnings

* add missing field for TestUser init

tests/utils/fixture_schemas.py

@@ -7,6 +7,7 @@ from uuid import UUID
 class TestUser:
     email: str
     user_id: UUID
+    username: str
     _group_id: UUID
     token: Any