pkgctl: Support automatic GitLab token creation

Currently if you run pkgctl auth login the browser is opened and you need to create the token manually in the UI and then copy it to the terminal. This is annoying.

My understanding is that SSH is often already configured by the user for our GitLab instance, which means we can use GitLab Shell personal_access_token command to get a new token with SSH.

Basically we just need to run:

$ ssh git@gitlab.archlinux.org personal_access_token pkgctl api,write_repository 30
Token:   glpat-XXXXXXXXXXXXXXXXXXXX
Scopes:  api,write_repository
Expires: 2025-02-05

Implementing some kind of token rotation (#150), should also be rather trivial.