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

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

services/context/repo.go

@@ -101,7 +101,7 @@ type CanCommitToBranchResults struct {
 	UserCanPush       bool
 	RequireSigned     bool
 	WillSign          bool
-	SigningKey        string
+	SigningKey        *git.SigningKey
 	WontSignReason    string
 }