allow startWith to take observable as input

[mikelgo]

HI,
in a lot of cases it would be quite handy that startWith could take another observable as initial value instead of just a simple value.

Example

const startValue$ = of(anyThing)

const source$ = some$.pipe(
  startWith(startValue$)
)

Does this feature request makes sense for you? Looking forward to feedback

[voliva]

const source$ = concat(startValue$, some$)