Add padding to compressed+encrypted files (#15282)

Add up to 256 bytes of padding for compressed+encrypted files.

This will obscure the obvious cases of extremely compressible content 
and leave a similar output size for a very wide variety of inputs.

This does *not* mean the compression ratio doesn't leak information 
about the content, but the outcome space is much smaller, 
so often *less* information is leaked.

cmd/object-api-utils_test.go

@@ -624,7 +624,7 @@ func TestS2CompressReader(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			buf := make([]byte, 100) // make small buffer to ensure multiple reads are required for large case
 
-			r, idxCB := newS2CompressReader(bytes.NewReader(tt.data), int64(len(tt.data)))
+			r, idxCB := newS2CompressReader(bytes.NewReader(tt.data), int64(len(tt.data)), false)
 			defer r.Close()
 
 			var rdrBuf bytes.Buffer