Add support for compression-dictionary-transport

[kkmuffme]

See kjdev/php-ext-zstd#73

[kkmuffme]

https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-compression-dictionary#name-dictionary-compressed-brotl
https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Content-Encoding#dcb

[kkmuffme]

Implementing the brotli dictionary generator https://github.com/google/brotli/blob/master/research/dictionary_generator.cc (as used on e.g. https://use-as-dictionary.com/generate/) would be nice too. e.g. passing an array of texts and the dictionary size as params to generate a dictionary
Since https://use-as-dictionary.com/generate/ is in PHP, there is perhaps already a PHP implementation of it and we could get that?

EDIT: this brotli dictionary thing would also work for zstd dictionary/zstd PHP, since it's in raw (= plain text) format

[kjdev]

d757244: experimental: dcb support in output handler

[kkmuffme]

Thanks, will test it next week.

Possibly addition to docs that custom dictionaries only work from level 5 or higher google/brotli#1148

[kjdev]

d89b21e added notes about dictionary