feat: dnsping using step-by-step (#831)

Reference issue for this pull request: https://github.com/ooni/probe/issues/2159

This diff refactors the `dnsping` experiment to use the [step-by-step measurement style](https://github.com/ooni/probe-cli/blob/master/docs/design/dd-003-step-by-step.md).

Co-authored-by: decfox <decfox@github.com>
Co-authored-by: Simone Basso <bassosimone@gmail.com>

internal/engine/experiment/dnsping/dnsping_test.go

@@ -50,7 +50,7 @@ func TestMeasurer_run(t *testing.T) {
 		if m.ExperimentName() != "dnsping" {
 			t.Fatal("invalid experiment name")
 		}
-		if m.ExperimentVersion() != "0.1.0" {
+		if m.ExperimentVersion() != "0.2.0" {
 			t.Fatal("invalid experiment version")
 		}
 		ctx := context.Background()