fix: model_fields attribute access warning in connector.py

[Fixemega]

Fixed warning about model_fields access being deprecated on Pydantic v2.11+ in connector.py by accessing it through the model class using __class__.

Example warning:

../../unstructured_ingest/interfaces/connector.py:35
PydanticDeprecatedSince211: Accessing the 'model_fields' attribute on the instance is deprecated.
Instead, you should access this attribute from the model class.
Deprecated in Pydantic V2.11 to be removed in V3.0.
    access_config_type = self.model_fields["access_config"].annotation