enhancement: add signoff option in commit form (#14516)

Signed-off-by: a1012112796 <1012112796@qq.com>

modules/structs/repo_file.go

@@ -17,6 +17,8 @@ type FileOptions struct {
 	Author    Identity          `json:"author"`
 	Committer Identity          `json:"committer"`
 	Dates     CommitDateOptions `json:"dates"`
+	// Add a Signed-off-by trailer by the committer at the end of the commit log message.
+	Signoff bool `json:"signoff"`
 }
 
 // CreateFileOptions options for creating files