Consider exposing the current page token in `ItemPaginator`

[dbolduc]

ItemPaginator might want to return the current page token (so streams can be restarted). Exposing the current page is probably good enough.

[coryan]

We need to figure out if this would be a breaking change. Probably not, but if it was, then we need to do it as part of stabilizing the API.

[dbolduc]

We need to figure out if this would be a breaking change.

Adding a function would have been a breaking change, but only because I forgot to seal the thing. Now that #1971 is in, it won't be.