gateway: support for UnixFS 1.5 Last-Modified/If-Modified-Since with remote CAR backend

[lidel]

https://github.com/ipfs/boxo/pull/659 introduced support for returning optional UnixFS 1.5 mtime value as Last-Modified header, and returning 304 Not Modified if it is matching If-Modified-Since sent with a file request.

This

• :green_circle: works with gateway/backend_blocks.go (used by Kubo and Rainbow)
• :construction: but does not work with gateway/backend_car.go (used in boxo/gateway conformance tests here and in Rainbow with RAINBOW_REMOTE_BACKENDS_MODE=car)

Remaining work here:

• [ ] prerequisites
  • [ ] figure out what is missing in loadTerminalEntity from gateway/backend_car.go to be able to read mtime from the root dag-pb block of requested file, update any upstream libraries if necessary
• [ ] car backend
  • [ ] return Last-Modified on deserialized GET responses
  • [ ] return Last-Modified on deserialized HEAD responses
  • [ ] 304 Not Modified on matching If-Modified-Since (UnixFS 1.5 file only)
• [ ] add gateway-conformance test for unixfs 1.5 and release it as new major version
  • [ ] confirm conformance passes with all backends (remote car uses different implementation of Get and Head and ContentPathMetadata needs to be updated there as well -- this is why conformance tests need to be added after we have car backend implemented first)