refactor(client)!: refactor async auto-pagination
refactor(client)!: rename `getNextPage{,Params}` to `nextPage{,Params}`
refactor(client)!: swap `nextPage{,Params}` to return non-optional

# Migration

- If you were referencing the `AutoPager` class on a specific `*Page` or `*PageAsync` type, then you should instead reference the shared `AutoPager` and `AutoPagerAsync` types, under the `core` package
- `AutoPagerAsync` now has different usage. You can call `.subscribe(...)` on the returned object instead to get called back each page item. You can also call `onCompleteFuture()` to get a future that completes when all items have been processed. Finally, you can call `.close()` on the returned object to stop auto-paginating early
- If you were referencing `getNextPage` or `getNextPageParams`:
   - Swap to `nextPage()` and `nextPageParams()`
   - Note that these both now return non-optional types (use `hasNextPage()` before calling these, since they will throw if it's impossible to get another page)

There are examples and further information about pagination in the readme.

Refactor `HttpRequest` to always take a `baseUrl`, instead of storing this in `OkHttpClient`. This allows better reuse of `OkHttpClient` when changing the `baseUrl`.

…is gc'd