`delayWhen`'s `delayDurationSelector` should support `ObservableInput`

[jakovljevic-mladen]

Discussed in #6972

^{Originally posted by jollytoad May 12, 2022}
Some operators such as bufferWhen, windowWhen, audit allow their selector fn to return an ObservableInput which really helps with interop with native JS features, such as Promises, Iterables/AsyncIterables etc.

But the delayWhen delayDurationSelector fn must return an Observable, it would really help if this was also ObservableInput. I'm not sure if this is intentional (I don't understand why it would be), or an oversight. I'm not sure if this is the case elsewhere, but it would certainly help if it was consistent across all selector fns.