Allow path parameters to be specified at router level

[kaschnit]

This PR proposes and implements the following.

Current behavior

Adding path params when including the router (e.g. app.add_router("/item/{item_id}", router) does not work as expected. The test client and the actual application routing logic cannot find the endpoint.

New (proposed) behavior

Using app.add_router("/item/{item_id}", router) will allow the user to treat item_id as a path parameter if they so choose.

This change should not break any existing behavior.

Example

With FastAPI, you can do something like this

api = FastAPI()
router = Router(prefix="item/{item_id}/")

@router.get("/metadata")
def get_item_metadata(item_id: str):
  return item_id

api.include_router(router)

Then you could do the following and expect output:

# command
curl localhost:8000/item/my_item/metadata

# output
my_item

This change will allow a similar pattern in django-ninja path parameters when including the router in a parent router, e.g.:

app = Ninja()
router = Router()

@router.get("/metadata")
def get_item_metadata(request, item_id: str = Path(None)):
  return item_id

app.add_router("/item/{item_id}/", router)

This PR only allows this to work when using the Path() object to indicate that it's a path parameter. The change to make it auto detect these is less trivial because the router's "prefix" is not actually a property of the Router, but a property of its parent router, so reading the prefix path is difficult when resolving path parameters.

[vitalik]

thank you!