add demo mode to skip greetd socket dependency

[rtgiskard]

a try to resolve: #30

[rharish101]

Thanks a lot! However, it's still incomplete: the greeter just stops when one clicks the login button. I think that there should be a way to also set fake users/logins, so that we can test all common greetd behaviours.

[rtgiskard]

Well, it's almost done, the demo auth verify predefined OTP and password in sequence (you may input password for otp, and the auth will pass directly), which should be fine for a demo.

[rharish101]

Thanks for the PR. Sorry for the late merge!