Support context cancellation in Gateway directory listings

[iand]

Checklist

• My issue is specific & actionable.
• I am not suggesting a protocol enhancement.
• I have searched on the issue tracker for my issue.

Description

This is a low priority general enhancement to conserve resources when dealing with slow clients. Directory listings may be very large and take a long time to transfer and we should respect cancellation of the request context (due to timeout or explicit).

As of commit 862ce6b, directory listings iterate over an Ls of a UnixFS and do not check for context cancellation:

https://github.com/ipfs/go-ipfs/blob/862ce6bb8f6ce24b91ed3dc59e7406faed34f583/core/corehttp/gateway_handler_unixfs_dir.go#L117

The request context is available and could simply be checked during iteration