public abstract Task <IActionResult> SendToConnection(string hub, string connectionId, [FromBody] PayloadMessage message);
 public abstract Task <IActionResult> GroupBroadcast(string hub, string group, [FromBody] PayloadMessage message, [FromQuery(Name = "excluded")] IReadOnlyList <string> excluded);
 public abstract Task <IActionResult> SendToUser(string hub, string user, [FromBody] PayloadMessage message);