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

close #15373

routers/api/v1/api.go

@@ -953,6 +953,7 @@ func Routes() *web.Route {
 					m.Get("/trees/{sha}", context.RepoRefForAPI, repo.GetTree)
 					m.Get("/blobs/{sha}", context.RepoRefForAPI, repo.GetBlob)
 					m.Get("/tags/{sha}", context.RepoRefForAPI, repo.GetAnnotatedTag)
+					m.Get("/notes/{sha}", repo.GetNote)
 				}, reqRepoReader(models.UnitTypeCode))
 				m.Group("/contents", func() {
 					m.Get("", repo.GetContentsList)