Referential Actions Enum Class (#735)

* feat: add action enum class to referential actions

* feat: write validation func for action name string

* test: write test for validation referential action

* fix: backend database running for action test

* fix: set the string type of value enum class

* fix: debuging return statement type for validation

* fix: return non empty for empty action

* refactor: change in line return if statement

* fix: add iterate method in read document md

* fix: update foreign key docstring types

* docs: write documention of refernal actions

* docs: complete referential actions descriptions

* refactor: rename and reposition referential action

* refactor: change validate referential action func

* test: add assert check for really deleted rows

* fix: debug error problem in renamed enum class

* fix: apply black formatted codes

* docs: update the document for referential actions

* docs: added note for server default argument

Co-authored-by: collerek <collerek@gmail.com>

docs/queries/read.md

@@ -6,6 +6,7 @@ Following methods allow you to load data from the database.
 * `get_or_create(_defaults: Optional[Dict[str, Any]] = None, *args, **kwargs) -> Tuple[Model, bool]`
 * `first(*args, **kwargs) -> Model`
 * `all(*args, **kwargs) -> List[Optional[Model]]`
+* `iterate(*args, **kwargs) -> AsyncGenerator[Model]`
 
 
 * `Model`