Add startup route setup

[danton267]

Add the ability for packages to add routes that will be set up at the end of Dash init.

example:

my-package.py

from dash import Dash

def my_route_f():
   return {} # serializable object

Dash.add_startup_route("my-package-route", my_route_f, ["POST"])

_dash_startup_route/my-package-route will be created at the app=Dash(), and can be then used by the package features

[T4rk1n]

💃