This adds an optimized io.ReadAll that uses a bytes.Buffer + io.Copy
internally, improving the buffer growth ratio over the runtime append
and taking advange of io.WriterTo when available.

The only scenario where this optimizes version is slower than io.ReadAll is
when reading < 512 bytes of a reader that does not implement io.WriterTo.

For now I only updated tests to use the new function as updating the code
requires more careful consideration.

While at it, migrate from the deprecated ioutil.ReadAll to io.ReadAll.

goos: linux
goarch: amd64
pkg: github.com/elastic/elastic-agent-libs/iobuf
cpu: Intel(R) Xeon(R) CPU E5-2697A v4 @ 2.60GHz
BenchmarkReadAll/size_32B/io.ReadAll/WriterTo-32         	 7479658	       162.4 ns/op	     512 B/op	       1 allocs/op
BenchmarkReadAll/size_32B/io.ReadAll/Reader-32           	 7000012	       169.3 ns/op	     512 B/op	       1 allocs/op
BenchmarkReadAll/size_32B/ReadAll/WriterTo-32            	10630838	       112.3 ns/op	     112 B/op	       2 allocs/op
BenchmarkReadAll/size_32B/ReadAll/Reader-32              	 2111246	       567.3 ns/op	    1584 B/op	       3 allocs/op
BenchmarkReadAll/size_64B/io.ReadAll/WriterTo-32         	 7154652	       163.2 ns/op	     512 B/op	       1 allocs/op
BenchmarkReadAll/size_64B/io.ReadAll/Reader-32           	 7223264	       166.1 ns/op	     512 B/op	       1 allocs/op
BenchmarkReadAll/size_64B/ReadAll/WriterTo-32            	10400562	       113.5 ns/op	     112 B/op	       2 allocs/op
BenchmarkReadAll/size_64B/ReadAll/Reader-32              	 2129949	       558.7 ns/op	    1584 B/op	       3 allocs/op
BenchmarkReadAll/size_512B/io.ReadAll/WriterTo-32        	 2843871	       419.1 ns/op	    1408 B/op	       2 allocs/op
BenchmarkReadAll/size_512B/io.ReadAll/Reader-32          	 2871580	       413.1 ns/op	    1408 B/op	       2 allocs/op
BenchmarkReadAll/size_512B/ReadAll/WriterTo-32           	 4976233	       241.5 ns/op	     560 B/op	       2 allocs/op
BenchmarkReadAll/size_512B/ReadAll/Reader-32             	 2183186	       552.9 ns/op	    1584 B/op	       3 allocs/op
BenchmarkReadAll/size_10KB/io.ReadAll/WriterTo-32        	  142633	      8235 ns/op	   46080 B/op	      10 allocs/op
BenchmarkReadAll/size_10KB/io.ReadAll/Reader-32          	  148326	      8229 ns/op	   46080 B/op	      10 allocs/op
BenchmarkReadAll/size_10KB/ReadAll/WriterTo-32           	  574903	      2210 ns/op	   10288 B/op	       2 allocs/op
BenchmarkReadAll/size_10KB/ReadAll/Reader-32             	  147210	      7995 ns/op	   32304 B/op	       7 allocs/op
BenchmarkReadAll/size_100KB/io.ReadAll/WriterTo-32       	   13171	     90853 ns/op	  514304 B/op	      18 allocs/op
BenchmarkReadAll/size_100KB/io.ReadAll/Reader-32         	   12892	     91787 ns/op	  514304 B/op	      18 allocs/op
BenchmarkReadAll/size_100KB/ReadAll/WriterTo-32          	   51472	     22420 ns/op	  106544 B/op	       2 allocs/op
BenchmarkReadAll/size_100KB/ReadAll/Reader-32            	   21568	     55070 ns/op	  261680 B/op	      10 allocs/op
BenchmarkReadAll/size_1MB/io.ReadAll/WriterTo-32         	    1220	    983276 ns/op	 5241098 B/op	      27 allocs/op
BenchmarkReadAll/size_1MB/io.ReadAll/Reader-32           	    1089	    990818 ns/op	 5241100 B/op	      27 allocs/op
BenchmarkReadAll/size_1MB/ReadAll/WriterTo-32            	    4153	    294507 ns/op	 1048627 B/op	       2 allocs/op
BenchmarkReadAll/size_1MB/ReadAll/Reader-32              	    1195	    944781 ns/op	 4193852 B/op	      14 allocs/op