7zip reader: add support for zstandard compression in 7z archives

7zip reader: add support for zstandard compression in 7z archives

Zstandard support is not yet available in 7-Zip, though it is planned
for a future release:
https://sourceforge.net/p/sevenzip/feature-requests/1580/

The compression ID used here (4F71101) is copied from a popular 7-Zip
fork, which added ZSTD support back in 2017, and is mentioned in the
upstream 7-Zip ticket linked above:
https://github.com/mcmilk/7-Zip-zstd.git

Note that this does not add write support for Zstandard compression
in 7z archives.

Relates to #1656.