Gateway directory listings should be paginated

### Checklist

- [X] My issue is specific & actionable.
- [X] I am not suggesting a protocol enhancement.
- [X] I have searched on the [issue tracker](https://github.com/ipfs/go-ipfs/issues?q=is%3Aissue) for my issue.

### Description

To render the directory listing page, `go-ipfs` sequentially fetches blocks for every directory entry. For large directories, this takes a very long time (see e.g. https://github.com/ipfs/go-ipfs/issues/7588). The gateway should paginate this listing so that there's a reasonable upper bound on the time it takes to return a response, and to allow more even load distribution across gateway fleets. I'd suggest some query args to control page size and offset, with an upper bound of 20 on the page size (this upper bound could be configurable).