text: Title: [BUG] AttributeError: 'method' object has no attribute '_ninja_operation'
  Body: **Describe the bug**
  When I'm trying to create a class-based router using the ApiRouter class as a base class, I receive this error at the time of self.add_api_operations:
  
  ```
      view_func._ninja_operation = operation  # type: ignore
      ^^^^^^^^^^^^^^^^^^^^^^^^^^
  AttributeError: 'method' object has no attribute '_ninja_operation'
  ```
  
  When I comment out this line in the source code for django-ninja my project works absolutely correctly.
  
  Code snippet:
  
  ```
  class TestRouter(Router):
      def __init__(self: Self) -> None:
          super().__init__()
  
          self.tags = ["Router"]
  
          self.add_api_operation(
              methods=["POST"],
              path="/asd",
              view_func=self.hello,
          )
  
      def hello(self: Self, request: WSGIRequest) -> str:
          return "ok"
  ```
  
  **Versions (please complete the following information):**
   - Python version: 3.12.3
  
  Note you can quickly get this by runninng in `./manage.py shell` this line:
  ```
  >>> import django; import pydantic; import ninja; django.__version__; ninja.__version__; pydantic.__version__
  '5.1.3'
  '1.3.0'
  '2.10.4'
  ```
  