Add support for compression-dictionary-transport

[pmeenan]

Compression Dictionary Transport provides a standard way to negotiate and use compression dictionaries over HTTP.

Spec is here and it shipped in Chrome 130.

It should "mostly" work with the existing code except for a 40-byte stream header for dcz encoding that includes the hash of the dictionary that was used.

It would be useful to also have an optional param on zstd_compress_dict to generate dcz files with the header.

[kkmuffme]

https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-compression-dictionary#name-dictionary-compressed-zstan

to be used with Content-Encoding dcz https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Encoding#dcz

[kjdev]

5c6d757 experimental: dcz support in output handler