support app factory patten in CLI (#2157)

* support app factory patten in CLI

* Update sanic/__main__.py

* fix mypy errors

* Update mypy further

* Update sanic/utils.py

* Update sanic/utils.py

* support hypercorn/gunicorn style 'asgi.app:create_app()'

* add test for app factory

tests/fake/server.py

@@ -30,3 +30,7 @@ async def app_info_dump(app: Sanic, _):
 @app.after_server_start
 async def shutdown(app: Sanic, _):
     app.stop()
+
+
+def create_app():
+    return app