Add support for custom django fields (#1109)

Using exclusively `django.db.fields.Field.get_internal_type` to
determine which Pydantic field to use in the built schemas makes it
difficult to create custom django fields. This PR adds support for a new
method `get_schema_type`, which allows using `get_internal_type` for its
intended purpose (determining database column types, etc.), while at the
same time allowing specifying which Pydantic schema field to use.

docs/docs/guides/response/django-pydantic.md

@@ -147,9 +147,48 @@ def modify_data(request, pk: int, payload: PatchDict[GroupSchema]):
 
     for attr, value in payload.items():
         setattr(obj, attr, value)
-    
+
     obj.save()
 
 ```
 
 in this example the `payload` argument will be a type of `dict` only fields that were passed in request and validated using `GroupSchema`
+
+### Custom fields
+
+For each Django field it encounters, `ModelSchema` uses the default `Field.get_internal_type` method
+to find the correct representation in Pydantic schema. This process works fine for the built-in field
+types, but there are cases where the user wants to create a custom field, with its own mapping to
+Pydantic field. Consider the following (barebones) example field:
+
+```python
+class TranslatedTextField(models.JSONField):
+    description = "Translated string represented as a JSON field"
+    languages = {"en": _("English"), "fr": _("French")}
+
+    def formfield(self, **kwargs):
+        is_required = not self.blank
+        return TextDictField(
+            widget=TranslatedTextFieldWidget(languages=self.languages), required=is_required
+        )
+```
+
+In all cases where this field is used in a model, we would like to use the following schema:
+
+```python
+class TranslatedTextFieldSchema(Schema):
+    en: Optional[str] = None
+    fr: Optional[str] = None
+```
+
+To achieve that, we can add a `get_schema_type` function to the django field and add it to supported
+types in Django Ninja:
+
+```python
+class TranslatedTextField(models.JSONField):
+    ...
+    def get_schema_type(self):
+        return "TranslatedTextField"
+
+ninja.orm.fields.TYPES["TranslatedTextField"] = TranslatedTextFieldSchema
+```

ninja/orm/fields.py

@@ -147,8 +147,15 @@ def get_schema_field(
         null = field_options.get("null", False)
         max_length = field_options.get("max_length")
 
-        internal_type = field.get_internal_type()
-        python_type = TYPES[internal_type]
+        if hasattr(field, 'get_schema_type'):
+            internal_type = field.get_schema_type()
+        else:
+            internal_type = field.get_internal_type()
+
+        try:
+            python_type = TYPES[internal_type]
+        except KeyError as e:
+            raise KeyError("Type '{0}' isn't registered in ninja.orm.fields.TYPES".format(internal_type)) from e
 
         if field.primary_key or blank or null or optional:
             default = None