Feat: Support ReadableStream as an ObservableInput

[benlesh]

Although RxJS gets a LOT of use in Node, RxJS's primary space has been in the browser. Based off of discussion in this Twitter thread about fetch streaming

We might want to support duck-typing ReadableStream<T> as an ObservableInput<T>.

If we did this, it might be beneficial, especially in the fetch API. But it's probably worth investigating what all use cases might exist.

I definitely think if it's added to from, schedule, and internally, innerFrom, it would need to be the absolute last thing checked. The presence of asyncIterator or iterator should definitely beat out a check for typeof o.getReader === 'function'.

This would be a lightweight and powerful add, IMO.

@jayphelps generously volunteered to do the work.

[jayphelps]

The presence of asyncIterator or iterator should definitely beat out a check for typeof o.getReader === 'function'.

Agreed.

---

I'll hold off until there is true consensus that this would want to be landed, since while it's not "hard" it's also not a 5 minute thing to PR.

[benlesh]

General approval from the core team 👍