Add an api endpoint to fetch git notes (#15373) (#16649)

close #15373

modules/structs/repo_note.go

@@ -0,0 +1,11 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package structs
+
+// Note contains information related to a git note
+type Note struct {
+	Message string  `json:"message"`
+	Commit  *Commit `json:"commit"`
+}