Embed hash of dictionary in compressed resource (optionally)

[pmeenan]

It would be useful if, when using a raw dictionary for compression, the compressor could embed a hash of the dictionary that was used during compression. Then, at decompression time, if the hash is present and doesn't match the provided dictionary the decompression would fail. It could also be used to identify which dictionary in a set of dictionaries was used at decompression time.

For the HTTP content-encoding we are currently passing the dictionary hash as an additional response header but that depends on the header and the resource always being together. It would be cleaner if the payload itself could contain the dictionary information.

See httpwg/http-extensions#2770

[pmeenan]

Closing in favor of #1167