get APP_CONFIG_KEY from parent app

Hello,
if we use init CORS in main app not work in subapp
```Traceback (most recent call last):
File aiohttp_cors/mixin.py", line 22, in get_request_config
cors = request.app[APP_CONFIG_KEY]
File aiohttp/web_app.py", line 104, in __getitem__
return self._state[key]
KeyError: 'aiohttp_cors'
```

fix problem use config_dict https://docs.aiohttp.org/en/stable/web_reference.html#aiohttp.web.Request.config_dict

`cors = request.config_dict[APP_CONFIG_KEY]`

thanks!