Draft: Support static challenge auth

Hello

I want to implement static-challenge 2fa feature

Property in Openvpn config - static-challenge "Enter Authenticator Code" 1

This PR works, but have some problems (it cant ask nm to new static-challenge, if previous was wrong) so, it is not ready now.

Fixes #98

Some questions

1. How 'need_secrets' signal works? How nm got what secret plugin need?
2. When i provide wrong static-challenge, nm removes the 'password' secret, and ask it again and again, i think it is because i didnt delete wrong static-challenge-response from context (Why nm always deleting password? How can i control this? I wanna delete static-challenge-response with it too). But if i provide correct, all working as i expected, and vpn connects

So, help me please :)