add functionality similar to fastapi.APIRouter.add_api_route

[kj4tmp]

Add a function to nicegui.APIRouter() that allows page declaration without using a decorator.

function could be something like nicegui.APIRouter.add_page_route

reason being this allows a page function to be declared within a class, and the page function would have access to self.

reference

[rodja]

Sounds good. Anyone interested in creating a pull request?