pull: Add tls-client-cert-{path,key} (if we have new enough libsoup)

This is an actually working version of client-side certificates.
Depends on:
See: https://bugzilla.gnome.org/show_bug.cgi?id=334021

We detect whether libsoup is new enough for this.

https://bugzilla.gnome.org/show_bug.cgi?id=729356