gateway: add accept-ranges response header

[lidel]

Many CDNs (incl. Cloudflare) won't support range requests of big files (>1-4GiB) if origin HTTP server does not return explicit support for range requests indicated by Accept-Ranges: bytes in GET responses.

This should apply to deserialized, deterministic response types.

TODO

• boxo/gateway fix (GET and investigate HEAD)
• include in Kubo 0.34
• explicit gateway-conformance tests: Add tests for HTTP Range requests gateway-conformance#154

[lidel]

Seems that something else is at play. I've checked multiple times and both GET and HEAD for 20GiB file include accept-ranges: bytes in response already.

Pausing until we get some clarifications from Cloudflare.