[Suggestion] Support OpenAPI 3.1 webhooks in code via @api.webhook(...) or similar

In my projects I often need to define the payload/schema for webhooks, and OpenAPI 3.1 supports this via "webhooks" (duh...). I would suggest adding a more convenient support for this to django-ninja:

from ninja import NinjaAPI, Schema

class Pet(Schema):
    id: int
    name: str

api = NinjaAPI()

@api.webhook("petCreated")
def pet_created(request, payload: Pet):
    """Fired when a pet is added to the catalog."""
    # Never called - signature is read only to derive the requestBody.

Renders in Swagger UI as a top-level Webhooks section, with Pet shared in components.schemas:

I can imagine a few different approaches to this, so let me know if there is any preference for something else.

I will open a PR in a second to show better what I mean.