feat: add tool to send a test notification (#16611)

Relates to https://github.com/coder/coder/issues/16463

Adds a CLI command, and API endpoint, to trigger a test notification for
administrators of a deployment.

coderd/coderd.go

@@ -1370,6 +1370,7 @@ func New(options *Options) *API {
 				r.Get("/system", api.systemNotificationTemplates)
 			})
 			r.Get("/dispatch-methods", api.notificationDispatchMethods)
+			r.Post("/test", api.postTestNotification)
 		})
 		r.Route("/tailnet", func(r chi.Router) {
 			r.Use(apiKeyMiddleware)