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-expected.conf

@@ -1,3 +1,12 @@
+[commit]
+	gpgSign = false
+
+[gpg]
+	format = "openpgp"
+
+[gpg "openpgp"]
+	program = "path-to-gpg"
+
 [sendemail "hm-account"]
 	from = "H. M. Test Jr. <hm@example.org>"
 	smtpEncryption = "tls"
@@ -12,6 +21,9 @@
 	smtpSslCertPath = "/etc/ssl/certs/ca-certificates.crt"
 	smtpUser = "home.manager"
 
+[tag]
+	gpgSign = false
+
 [user]
 	email = "hm@example.com"
 	name = "H. M. Test"