gzhttp: Add zstd to transport

[klauspost]

goos: windows
goarch: amd64
pkg: github.com/klauspost/compress/gzhttp
cpu: AMD Ryzen 9 3950X 16-Core Processor            
BenchmarkTransport
BenchmarkTransport/gzhttp-32         	    2179	    503006 ns/op	 259.61 MB/s	        25.67 pct	    9623 B/op	      74 allocs/op
BenchmarkTransport/stdlib-32         	    2001	    596271 ns/op	 219.00 MB/s	        25.67 pct	   52275 B/op	      92 allocs/op
BenchmarkTransport/zstd-32           	    3404	    343757 ns/op	 379.87 MB/s	        24.44 pct	    5358 B/op	      69 allocs/op
BenchmarkTransport/gzhttp-par-32     	   47127	     25402 ns/op	5140.75 MB/s	        25.67 pct	    9598 B/op	      72 allocs/op
BenchmarkTransport/stdlib-par-32     	   39920	     30834 ns/op	4235.03 MB/s	        25.67 pct	   52269 B/op	      90 allocs/op
BenchmarkTransport/zstd-par-32       	   68941	     17277 ns/op	7558.02 MB/s	        24.44 pct	    5436 B/op	      67 allocs/op

PASS

Process finished with the exit code 0

• Tests added.