[Housekeeping]: Use JSONField for assigned_object serializer

[pheus]

Proposed Changes

• Replace DictField with JSONField for the assigned_object serializer.
• Ensure the schema field returns None when assigned_object is missing, without causing errors.
• Add a string representation for the "display" attribute in the ACLInterfaceAssignment API.

Justification

• Since this is a REST API, using JSONField is more appropriate and aligns with NetBox core best practices.
• The "display" attribute is a common pattern in the NetBox API, so it should be included in ACLInterfaceAssignment for consistency.