Feature request: Add `Flow#switchOnFirst` operator for shortcut of `prefixAndtail(1).flatmapConcat(..)` ?

Motivation:
I find it's very common to write code as `prefixAndTail(1).flatMapConcat(...)`, this kind is just something like `Flux#switchOnFirst`.
And more fine control can be done with the `prefixAndTail` but I think for most case is `prefixAndTail(1).flatMapConcat(...)` like thing.

Result:
Simpler code for common use case.