fix: forkJoin POJO signature should match only ObservableInput values

[cartant]

Description:

This PR tightens the forkJoin POJO signature so that only POJOs with ObservableInput values are matched. The implementation throws if the received POJO contains a value that is not an ObservableInput, so the types should reflect that.

As an aside, we should look at renaming some of our utility types, as the ObservableValueTuple type is mapped type that's not restricted to tuples. It could be used in this signature, too:

export function forkJoin<T extends Record<any, ObservableInput<any>>>(
  sourcesObject: T
): Observable<ObservableInputTuple<T>>;

But having Tuple in the type name makes no sense here. We can do this later, after v7, and can deprecate the old names (which have been exported).

Related issue (if exists): Nope