Enable registering callback when future completes

[HaloFour]

Adds a SiriusFuture[T] interface which adds addListener methods allowing a consumer to register a Runnable callback for when the underlying Akka Future[T] completes. This would enable basic asynchronous composition without having to block the current thread or poll the Future[T] for completion.

This could be a breaking change for any consumer that directly implements the Sirius interface as the return value has been changed from Future[T] to SiriusFuture[T], but consumers of the interface should be fine as SiriusFuture[T] extends Future[T].

[CLAassistant]

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