Add `Shared Brotli` and `Shared Zstandard` (zstd) about server-configs-apache HOT 2 OPEN

Thanks for this sharing @summercms.
That said, I'm not sure what can be done with H5BP configs regarding this (still to be published) standard. Are we sure this is compatible with current server software?

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@LeoColomb

See slide 4 found here: https://datatracker.ietf.org/meeting/117/materials/slides-117-httpbis-compression-dictionaries-00

Here's what I understand:

Use-As-Dictionary: match="/path/to/target/*"

When the client Use-As-Dictionary transitions to the path specified in , it adds to the request header that it supports CDT in Brotli ( sbr) and has a dictionary that can be applied there, as shown below.

Accept-Encoding: sbr
Sec-Available-Dictionary: <SHA-256 of Dictionary>

The server can use the dictionary specified by the client to compress and return the content.

If compression is performed using a dictionary, the response will be as follows. Since the encoding result of the same content changes depending on the dictionary, add the hash of the dictionary to the key of Sec-Available-Dictionary Cache Vary.

Content-Encoding: sbr
Vary: Accept-Encoding, Sec-Available-Dictionary

Since it is a method that generates a dictionary in advance, it can be said to be a method that is compatible with static site generators.

Was thinking could update this code section, with notes at a later date:

Disclaimer: This spec is likely to change still, so expecting this to be put on hold for the next major semantic version of this repo.

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