iroh-gateway spec compatibility

[Arqu]

Originally we took the go-ipfs (now kubo) implementation of the gateway as the baseline of what's expected from a gateway. Since then this spec came out and we should aim to comply with it as much as possible (which will also lead us to sand down some of our rough edges).

From the spec so far we're missing:

• HEAD support
• Proper ETag and Cache control handling
• Support for Range header/reqs
• Review Accept-Headers
• Double check Service-Worker handling
• Align/simplify request url ?params
• Audit response headers
• Adjust responses and response codes to be in line with spec

Issues we already have that cross over into it or are appropriate improvements:

• Clarify expected cache headers #190
• gateway(paths): use resolver::Path instead of stringified version where possible #248
• gateway(requests): handle HEAD requests #254
• better mime type detection #203
• gateway(encoding): support utf-8 #252
• feat(gateway): add support for range requests #234
• gateway(unixfs): improve dir listing #258
• https://github.com/n0-computer/iroh/issues/97
• improve error handling #261

Punting due to other priorities:

• gateway(etag): handle dir etags on gateway #257
• gateway(cache): improve cache control headers #251

Potential extensions:

• IPIP-288: TAR Gateway Response Format ipfs/specs#288

[Arqu]

range reqs n0-computer/iroh#330

[Arqu]

Missing 404 handling / templates for completion.

[Arqu]

closing in favor of #89