bug#71678: Support git commits signed with ssh keys in guix git authenticate

[MSavoritias]

Hey,

I sign all my commits and tags and such in git with SSH keys and I
would like to be able to use `guix git authenticate` to have a secure
channel pull mechanism.

After doing some testing myself and reading the guide here
https://guix.gnu.org/manual/devel/en/html_node/Specifying-Channel-Authorizations.html
it seems Guix doesn't support git commits that have been signed with
SSH keys.

Personally for security reasons mainly I do not use gpnupg and ssh
support has been in git for a while. Namely November 2021.
https://lore.kernel.org/git/xmqq8rxpgwki.fsf@gitster.g/

Reading libgit2 issues it seems it is not supported yet
https://github.com/libgit2/libgit2/issues/6397
So lets circle back to this when it actually merges support :D

Regards,
MSavoritias