Expose an isObservableInput function

[csisy-bt4w]

Given the following function overload example:

function foo(source: ObserableInput<unknown>): void;
function foo(source: Record<string, ObserableInput<unknown>>): void;

Currently, it is inconvenient to write the implementation, since there is no easy way to check if an argument is an ObservableInput.

However, the code itself is already somewhat implemented here:

rxjs/packages/observable/src/observable.ts

Line 1241 in f9f07f1

export function getObservableInputType(input: unknown): ObservableInputType {

I propose to expose a type guard function isObservableInput similarly to isObservable.