Add Nimiq Auth

[code3z]

Basically just signing a message that says “Login at for ” but using a UI that says “login” or “authorize app” not “sign message.” Use case: user signs in to access benefits for purchasing something.

[sisou]

This has been explored and proposed in the past as NimiqID, I even wrote a whole RFC for it:
https://github.com/nimiq/rfcs/blob/nimiq-id/accepted-rfcs/0000-nimiq-id.md

Sadly, it never got any comments:
nimiq/rfcs#1