QtNetwork should support zstd content encoding (rfc8478)

Since Qt 5.13 has instroduced dcependency on libzstd [1] (which is used now as a default compression algorithm for QResource, thanks thiago), it becomes possible to support zstd as HTTP content-encoding type [2] in QNetworkAccessManager. It's already supported by https://www.facebook.com/, and everyone can use it on server side via Nginx module [3].

[1] https://facebook.github.io/zstd/
[2] https://tools.ietf.org/html/rfc8478#section-6.2
[3] https://github.com/tokers/zstd-nginx-module