Implement SSH GcrImporter
Seahorse, which is based on gcr, can't import SSH keys (and possibly others) apparently due to this feature lacking in gcr. This is an apparently very old issue and quite important for some people as pointed out in this report: seahorse#205
Issue exists on gcr 3.28.1 and Seahorse 3.30.1 on Archlinux updated to the time of this report.