Support decompression of stacked compressed responses

[na--]

Currently, k6 supports compression of request bodies with multiple layered algorithms like gzip, deflate. However, it doesn't support decompressing response bodies that are compressed in a similar manner, which is arguably the more important use-case.

https://github.com/loadimpact/k6/blob/350729e053ac36c09bb26c11e4c7fb87b909f597/lib/netext/httpext/request.go#L251-L276

I've also tagged this issue as refactoring, since the compression/decompression code should probably be moved to its own file, given how it's currently interspersed very annoyingly among the rest of the request handling code...

https://github.com/loadimpact/k6/blob/350729e053ac36c09bb26c11e4c7fb87b909f597/lib/netext/httpext/request.go#L73-L91

https://github.com/loadimpact/k6/blob/350729e053ac36c09bb26c11e4c7fb87b909f597/lib/netext/httpext/request.go#L118-L138

https://github.com/loadimpact/k6/blob/350729e053ac36c09bb26c11e4c7fb87b909f597/lib/netext/httpext/request.go#L148-L225