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/fixtures/fixture_admin.py

@@ -33,6 +33,7 @@ def admin_user(api_client: TestClient, api_routes: utils.AppRoutes):
         yield utils.TestUser(
             _group_id=user_data.get("groupId"),
             user_id=user_data.get("id"),
+            username=user_data.get("username"),
             email=user_data.get("email"),
             token=token,
         )