Subject and BehaviorSubject should have input and output types

[dariomannu]

Currently both Subject and BehaviorSubject have a single type parameter.

This is in sheer contrast with the typical use case of both, which is to transform inputs into outputs potentially of a different type via pipelines.

Both should really be typed like Subject<I, O=I> and BehaviorSubject<I, O=I>, where I is the input type, and O the output type.
If no pipeline is attached, O would be obviously equal to I, otherwise:

const s = new Subject<string, number>().pipe(
  map((str: string) => parseInt(str))
);

This concept is based on the idea from #7506 that Subject().pipe() should be itself of type Subject and BehaviorSubject().pipe() should be itself of type BehaviorSubject, unless... that's challenged by anyone?