Add db.models.fields.Field method stubs for custom Fields (#331)

Add stubs for: pre_save, get_db_prep_value, get_db_prep_save

Author: danifus

django-stubs/db/models/fields/__init__.pyi

@@ -108,7 +108,10 @@ class Field(RegisterLookupMixin, Generic[_ST, _GT]):
     def set_attributes_from_name(self, name: str) -> None: ...
     def db_type(self, connection: Any) -> str: ...
     def db_parameters(self, connection: Any) -> Dict[str, str]: ...
+    def pre_save(self, model_instance: Model, add: bool) -> Any: ...
     def get_prep_value(self, value: Any) -> Any: ...
+    def get_db_prep_value(self, value: Any, connection: Any, prepared: bool) -> Any: ...
+    def get_db_prep_save(self, value: Any, connection: Any) -> Any: ...
     def get_internal_type(self) -> str: ...
     # TODO: plugin support
     def formfield(self, **kwargs) -> Any: ...