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

mealie/schema/response/__init__.py

@@ -1,2 +1,3 @@
 # GENERATED CODE - DO NOT MODIFY BY HAND
 from .responses import *
+from .validation import *

mealie/schema/response/validation.py

@@ -0,0 +1,5 @@
+from pydantic import BaseModel
+
+
+class ValidationResponse(BaseModel):
+    valid: bool = False