feat(sec): Add SSH signing support for instances (#6897)

- Add support to set `gpg.format` in the Git config, via the new `[repository.signing].FORMAT` option. This is to tell Git that the instance would like to use SSH instead of OpenPGP to sign its commits. This is guarded behind a Git version check for v2.34.0 and a check that a `ssh-keygen` binary is present.
- Add support to recognize the public SSH key that is given to `[repository.signing].SIGNING_KEY` as the signing key by the instance.
- Thus this allows the instance to use SSH commit signing for commits that the instance creates (e.g. initial and squash commits) instead of using PGP.
- Technically (although I have no clue how as this is not documented) you can have a different PGP signing key for different repositories; this is not implemented for SSH signing.
- Add unit and integration testing.
  - `TestInstanceSigning` was reworked from `TestGPGGit`, now also includes testing for SHA256 repositories. Is the main integration test that actually signs commits and checks that they are marked as verified by Forgejo.
  - `TestParseCommitWithSSHSignature` is a unit test that makes sure that if a SSH instnace signing key is set, that it is used to possibly verify instance SSH signed commits.
  - `TestSyncConfigGPGFormat` is a unit test that makes sure the correct git config is set according to the signing format setting. Also checks that the guarded git version check and ssh-keygen binary presence check is done correctly.
  - `TestSSHInstanceKey` is a unit test that makes sure the parsing of a SSH signing key is done correctly.
  - `TestAPISSHSigningKey` is a integration test that makes sure the newly added API route `/api/v1/signing-key.ssh` responds correctly.

Documentation PR: forgejo/docs#1122

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6897
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

routers/api/v1/api.go

@@ -865,6 +865,7 @@ func Routes() *web.Route {
 		m.Group("", func() {
 			m.Get("/version", misc.Version)
 			m.Get("/signing-key.gpg", misc.SigningKey)
+			m.Get("/signing-key.ssh", misc.SSHSigningKey)
 			m.Post("/markup", reqToken(), bind(api.MarkupOption{}), misc.Markup)
 			m.Post("/markdown", reqToken(), bind(api.MarkdownOption{}), misc.Markdown)
 			m.Post("/markdown/raw", reqToken(), misc.MarkdownRaw)