Improve instance wide ssh commit signing (#34341)

* Signed SSH commits can look in the UI like on GitHub, just like gpg keys today in Gitea
* SSH format can be added in gitea config
* SSH Signing worked before with DEFAULT_TRUST_MODEL=committer

`TRUSTED_SSH_KEYS` can be a list of additional ssh public key contents
to trust for every user of this instance

Closes #34329
Related #31392

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/repo_tree.go

@@ -15,7 +15,7 @@ import (
 type CommitTreeOpts struct {
 	Parents    []string
 	Message    string
-	KeyID      string
+	Key        *SigningKey
 	NoGPGSign  bool
 	AlwaysSign bool
 }
@@ -43,8 +43,13 @@ func (repo *Repository) CommitTree(author, committer *Signature, tree *Tree, opt
 	_, _ = messageBytes.WriteString(opts.Message)
 	_, _ = messageBytes.WriteString("\n")
 
-	if opts.KeyID != "" || opts.AlwaysSign {
-		cmd.AddOptionFormat("-S%s", opts.KeyID)
+	if opts.Key != nil {
+		if opts.Key.Format != "" {
+			cmd.AddConfig("gpg.format", opts.Key.Format)
+		}
+		cmd.AddOptionFormat("-S%s", opts.Key.KeyID)
+	} else if opts.AlwaysSign {
+		cmd.AddOptionFormat("-S")
 	}
 
 	if opts.NoGPGSign {