feat: Sysext Signing

[tulilirockz]

We could do something like we already do with signed containers with [cosign] but with our images! Even if that means just making a tarball with the image inside, it would be great to have proper signing. That would also require BEXT to support that signing and check if it is valid through some program like bext sign or something like that

This is a clone of #6 but worded in a different way, I literally didnt see this issue before, sorry