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

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](https://github.com/ipfs/kubo) and [Rainbow](https://github.com/ipfs/rainbow/))
- :construction: but does not work with `gateway/backend_car.go` (used in [boxo/gateway conformance tests here](https://github.com/ipfs/boxo/blob/3cd3857b046c3d26494c6e9a34dbcee707413648/.github/workflows/gateway-conformance.yml#L145-L150) and in Rainbow with [`RAINBOW_REMOTE_BACKENDS_MODE=car`](https://github.com/ipfs/rainbow/blob/main/docs/environment-variables.md#rainbow_remote_backends_mode))

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)