Add PublishRelay.asDriver()

[dmcyk]

This PR adds PublishRelay.asDriver() method.
Seems like PublishRelay is the only trait that's missing such conversion. All BehaviorRelay, ControlEvent and ControlProperty already implement asDriver.

[freak4pc]

Hey @dmcyk - Thanks for your contribution.
The fact Publish* cannot be turned into a Driver is not coincidental -

BehaviorRelay/BehaviorSubject model a State (hence it replays its latest value) and so does a Driver (models State).

PublishRelay/PublishSubject model Events (hence it does not replay latest value), but Driver is not intended for modeling events, as it requires replaying its latest event. The correct SharingStrategy for Publish* is actually a Signal - which is why asSignal is available for these units.

Hope this makes sense :)

Thanks again for the thought and assistance.
Shai.

[dmcyk]

oh I see. I wasn't aware of the separate Signal type Thanks for the explanation!