git: support alternate signing methods (#5516)

The Git module now supports SSH and X.509 signing in addition to
OpenPGP/GnuPG, via setting the `programs.git.signing.format` option.
It defaults to `openpgp` for now as a backwards compatibility measure,
but I feel like we shouldn't enforce GPG as the default on everyone,
especially for people who use SSH signing like me.

Accordingly, `programs.git.signing.gpgPath` has been renamed to
`programs.git.signing.signer`, as now the signer binary is not
restricted to GnuPG. Users should only get a warning and everything
should continue to work.

Fixes #4221, supersedes #4235

Co-authored-by: Mario Rodas <marsam@users.noreply.github.com>
Co-authored-by: Sumner Evans <me@sumnerevans.com>
Co-authored-by: Leah Amelia Chen <hi@pluie.me>

tests/modules/programs/git/git-with-email.nix

@@ -8,6 +8,7 @@
 
   programs.git = {
     enable = true;
+    signing.signer = "path-to-gpg";
     userEmail = "hm@example.com";
     userName = "H. M. Test";
   };