Support for zstd as response content-encoding and request body compression

[na--]

Info about Zstandard, i.e. zstd:

• https://facebook.github.io/zstd/
• https://en.wikipedia.org/wiki/Zstandard
• https://tools.ietf.org/html/rfc8478

Native Go library that seems to support it: https://github.com/klauspost/compress/tree/master/zstd#zstd

The brotli compression/decompression issue is very similar to this one: #776

After the refactoring in #989, it's now fairly easy to add support for new compression and content-encoding algorithms. Basically, if there's a nice Go library, we only need to plug it in these places (maybe with a bit of refactoring) and, of course, add some tests:
https://github.com/loadimpact/k6/blob/c85438949d00c2ca8a89d04b2837bb087a3d3201/lib/netext/httpext/request.go#L77-L91
https://github.com/loadimpact/k6/blob/c85438949d00c2ca8a89d04b2837bb087a3d3201/lib/netext/httpext/request.go#L142-L149
https://github.com/loadimpact/k6/blob/c85438949d00c2ca8a89d04b2837bb087a3d3201/lib/netext/httpext/request.go#L182-L198