Introduce a serializer field for GenericForeignKeys

[jeremystretch]

Proposed Changes

Devise and implement a custom serializer field to represent GenericForeignKeys on models. This is currently handled using a read-only SerializerMethodField, which requires a boilerplate method be included on the serializer class. For instance, the code below is from the serializer for circuit terminations:

    @extend_schema_field(serializers.JSONField(allow_null=True))
    def get_termination(self, obj):
        if obj.termination_id is None:
            return None
        serializer = get_serializer_for_model(obj.termination)
        context = {'request': self.context['request']}
        return serializer(obj.termination, nested=True, context=context).data

Justification

Introducing a custom field to represent GFK relationships allows us to enforce a standard implementation and reduce boilerplate.