[feature request] Add controlvalue$ function that adds the startingValue of a form to its value changes

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I often find myself to do the following:

const value$ = this.control.selectIndicator.valueChanges.pipe(startWith(this.control.selectIndicator.value));

Which can get quite lengthy especially when you have nested forms.

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Thats why I would propose a simple function that encapsulates this:

function controlvalue$<T>(control: AbstractControl<T>): Observable<T> {
  return control.valueChanges.pipe(startWith(control.value));
}

Would you be interested in such a function? I would do a PR then.
(Open for naming suggestions)