feat(coderd): add mark-all-as-read endpoint for inbox notifications (#16976)

[Resolve this issue](https://github.com/coder/internal/issues/506)

Add a mark-all-as-read endpoint which is marking as read all
notifications that are not read for the authenticated user.
Also adds the DB logic.

coderd/coderd.go

@@ -1395,6 +1395,7 @@ func New(options *Options) *API {
 			r.Use(apiKeyMiddleware)
 			r.Route("/inbox", func(r chi.Router) {
 				r.Get("/", api.listInboxNotifications)
+				r.Put("/mark-all-as-read", api.markAllInboxNotificationsAsRead)
 				r.Get("/watch", api.watchInboxNotifications)
 				r.Put("/{id}/read-status", api.updateInboxNotificationReadStatus)
 			})