feat: Support zstd encoding

[mrueg]

This allows endpoints to respond with zstd compressed metric data, if the requester supports it. For backwards compatibility, gzip compression will take precedence.

I added a benchmark to the http_test.go, that will test with a differently sized synthetic metric series.

Running tool: go test -benchmem -run=^$ -bench ^BenchmarkEncoding$ github.com/prometheus/client_golang/prometheus/promhttp

goos: linux
goarch: amd64
pkg: github.com/prometheus/client_golang/prometheus/promhttp
cpu: Intel(R) Core(TM) i7-1065G7 CPU @ 1.30GHz
BenchmarkEncoding/test_with_gzip_encoding_small-8         	   20670	     53641 ns/op	  111770 B/op	      49 allocs/op
BenchmarkEncoding/test_with_zstd_encoding_small-8         	    7838	    178033 ns/op	 1121493 B/op	      75 allocs/op
BenchmarkEncoding/test_with_no_encoding_small-8           	   56409	     20998 ns/op	   38030 B/op	      43 allocs/op
BenchmarkEncoding/test_with_gzip_encoding_medium-8        	    8493	    143460 ns/op	   78918 B/op	      49 allocs/op
BenchmarkEncoding/test_with_zstd_encoding_medium-8        	    3650	    345559 ns/op	 1155737 B/op	      76 allocs/op
BenchmarkEncoding/test_with_no_encoding_medium-8          	    8816	    122759 ns/op	   72252 B/op	      44 allocs/op
BenchmarkEncoding/test_with_gzip_encoding_large-8         	     944	   1206069 ns/op	  479829 B/op	      52 allocs/op
BenchmarkEncoding/test_with_zstd_encoding_large-8         	     817	   1482854 ns/op	 1429911 B/op	      75 allocs/op
BenchmarkEncoding/test_with_no_encoding_large-8           	     994	   1213586 ns/op	  346837 B/op	      45 allocs/op
BenchmarkEncoding/test_with_gzip_encoding_extra-large-8   	      85	  14008798 ns/op	 2972152 B/op	      56 allocs/op
BenchmarkEncoding/test_with_zstd_encoding_extra-large-8   	      90	  14638520 ns/op	 3731521 B/op	      74 allocs/op
BenchmarkEncoding/test_with_no_encoding_extra-large-8     	      93	  13897780 ns/op	 2648745 B/op	      45 allocs/op
PASS
ok  	github.com/prometheus/client_golang/prometheus/promhttp	17.534s

See also: prometheus/prometheus#13866