Helper to retrieve all pages

[cben]

Retrieving all pages is somewhat cumbersome - #11 - but for most programmatic uses it's the only correct way to use the API
(well, it's tempting to just request huge page size, but that's not really "correct", and there's been an idea that API might limit page size in future?)

So maybe the SDK could offer a helper that iterates all data.

• should it concatenate everything to one array? Or return some kind of iterator you can start consuming? Maybe a channel?

• requesting multiple pages in jumps risks missing/duplicating items if underlying collection is mutated in between. The SDK can't really solve this; but maybe we should think how to improve this on API side before abstracting a risky pattern in SDK...