Mobile push authenticator stage

**Is your feature request related to a problem? Please describe.**
I know there is support for Duo for having 2FA with mobile push notifications. Duo is paid service, which doesn't align with open source nature of Authentik.

**Describe the solution you'd like**
It would be great to have similar functionality of mobile push notifications, because rewriting OTP/HOTP codes are too user unfriendly compared to confirming notification on mobile phone, like eg Microsoft Authenticator is doing.

There is already complete mobile app developed by privacyIDEA project for their MFA server at https://github.com/privacyidea/pi-authenticator and published eg here https://play.google.com/store/apps/details?id=it.netknights.piauthenticator&hl=en&gl=US

Therefore it would be sufficient to publish that stage and compliant API as privacyIDEA server, or fork mobile app and create custom API.