Support awaiting signals on SignalCallable

[fuzzybinary]

futureSignal creates a one-time subscription using SignalAwaiter, but requires an object and a signal name. It would be good if we could support this directly on SignalCallable instead, and have it return a Dart record of the parameters sent to the signal.

As a one-time subscription, it should disconnect as soon as the signal is called, and properly 'cancel' the future if a disconnect is requested before the signal is received.