Gateway should generate an index for directories containing CBOR-DAGs

[dysbulic]

I am working on a git remote which, when a repo is pushed to IPFS, produces a UnixFS filesystem of the pushed branch with a .git CBOR-DAG object added.

When viewing the produced filesystem through the gateway, if it needs to generate an index, it fails with the error: internalWebError: unknown node type. If an index.html is present, that is served.

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[lidel]

Continued in #8223 (closing this one, as that one has more generic unixfs example)