Gateway with improved directory listing logic (Tsize-based)

[lidel]

Explainer: ipfs/kubo#9058 (I can create Kubo fork with the proposed change).

Would be good to clearly see the real-world benefit of the change.
To test this, we need to replay Gateway requests for UnixFS directories.
Should be possible to filter and find them: responses have Etag header starting with DirIndex.

[iand]

Scheduled as part of epic protocol/prodeng#25

[thattommyhall]

#55 will be useful for this, will need to change the tracing log too but thats dead simples.