Add `application/zstd` support for http requests/responses

[zstd](https://en.wikipedia.org/wiki/Zstd) is a new compression that is far superior to gzip/lzma in every way and it has also been published as https://www.rfc-editor.org/rfc/rfc8478 as `application/zstd` which means that http requests/responses can be compressed/decompressed via std.

In terms of browsers, Chromium and Firefox have already added support for zstd compression/decompression, which means that only Safari is needed to have support in all mainstream browsers.

Requires https://github.com/apache/pekko/issues/2404 to be completed and released, at which point we need to add the various header entries.