knotserver: verify signature for a few other endpoints

Thanks @dvjn.dev!

knotserver/handler.go

···
 	r.Route("/{did}", func(r chi.Router) {
 		// Repo routes
 		r.Route("/{name}", func(r chi.Router) {
-			r.Post("/collaborator/add", h.AddRepoCollaborator)
+			r.Route("/collaborator", func(r chi.Router) {
+				r.Use(h.VerifySignature)
+				r.Post("/add", h.AddRepoCollaborator)
+			})
 
 			r.Get("/", h.RepoIndex)
 			r.Get("/info/refs", h.InfoRefs)
 			r.Post("/git-upload-pack", h.UploadPack)
 
 			r.Route("/merge", func(r chi.Router) {
+				r.With(h.VerifySignature)
 				r.Post("/", h.Merge)
 				r.Post("/check", h.MergeCheck)
 			})