allowed_signers file for SSH/git

Git 2.34 brings support for signing commits with SSH key and having a SSH-compatible smartcard, I have to try this. It likely getting more common in the future doesn't hurt either and I have pgp-alt-wot which does about the same for PGP.

Where to find keys

• GitHub, Giteas and GitLabs expose user public keys when you append a .keys after their profile page
• Good ideas are made to be copied, so maybe there will be more repositories like this ;)

Further reading

• Caleb Hearth: Signing Git Commits with Your SSH Key, web.archive.org, inspired me to try this
• GitHub feedback: Allow using SSH keys to sign commits
  • TODO: notify here when it actually works, link to their guide or maybe remove the section?
• Merged Gitea PR for add support for ssh commit signing
  • TODO: link to their blog once it's released
• Fedora update request for git
  • TODO: remove this mention when it happens? It does have the test instructions I took