filter KIO: add zstd support

The docbook is a copy of the text for the other filter, and would be as wrong as they are ;)

The KArchive_HAVE_ZSTD as used as condition to build & install the docbook is yet missing from KArchive CMake config flag (-> frameworks/karchive!20 (merged)), though I found that the filter code itself does not use any of those conditions and always registers as handler for the related MIME types. Perhaps something to change in general.

Using the filter URLs from konqueror did not work in general (with any compression filters), somehow its handling of the MIME type reported fails to do what is expected. But using the filter with kioclient on zstd generated files e.g. by

kioclient5 exec zstd:/home/koder/print.pdf.zst

or

kioclient5 cat zstd:/home/koder/hello.txt.zst

worked, so I assume something needs fixing at konqueror?

Sadly seems Dolphin ignores the filter KIO slave completely, which is a shame :( Ark is nice, but why not being able to open compressed files directly, at least for users who are capable to get things...

Why stil this patch? Because completeness, as long as those KIO slaves exist, and it is simple to add.

BTW, the gzip filter seems to be broken (tested in comparison for text & pdf), something with skipping the header I guess from some first inspection, but have not yet fully understood the issue, perhaps you can overtake? The related KArchive code might be a matter of experts rather, the comments when to skip are too much for my late night brain ;) ).