watchTermination must have async closure version

[ktoso]

    @discardableResult
    public func watchTermination<Watchee>(
        of watchee: Watchee,
        @_inheritActorContext @_implicitSelfCapture whenTerminated: @escaping @Sendable (ID) -> Void,
        file: String = #file, line: UInt = #line
    ) -> Watchee where Watchee: DistributedActor, Watchee.ActorSystem == ClusterSystem {

this must be usable with an async whenTerminated