Add a request_certificate virtual method to GTlsInteraction

This allows GTlsConnection implementations to request a certificate
from the user.

https://bugzilla.gnome.org/show_bug.cgi?id=637257
fixup! Add a request_certificate virtual method to GTlsInteraction

https://bugzilla.gnome.org/show_bug.cgi?id=637257
fixup! Add a request_certificate virtual method to GTlsInteraction