[#170] Support tar.gz downloading

Split DownloadZip handler on methods. Add handler DownloadTar for downloading tar.gz archives. Make methods more universal for using in both implementations

Signed-off-by: Nikita Zinkevich <n.zinkevich@yadro.com>

internal/handler/handler_fuzz_test.go

@@ -517,7 +517,7 @@ func DoFuzzDownloadZipped(input []byte) int {
 	r.SetUserValue("cid", cid)
 	r.SetUserValue("prefix", prefix)
 
-	hc.Handler().DownloadZipped(r)
+	hc.Handler().DownloadZip(r)
 
 	return fuzzSuccessExitCode
 }