fix(webconnectivity): expose network events (#258)

* fix(webconnectivity): expose network events

By not exposing network events in webconnectivity, we are missing
several interesting, explanatory data points.

This diff fixes the issue by:

1. enriching the definition of network events to include extra
data useful for performing (manual) data analysis;

2. adding a tags field to network events such that we can add
tags to specific events and understand where they come from;

3. exposing all the (tagged) network events that happen when running
a webconnectivity experiment.

See https://github.com/ooni/probe-engine/issues/1157.

* progress

* more work towards landing this diff

* Apply suggestions from code review

internal/engine/experiment/webconnectivity/webconnectivity_test.go

@@ -21,7 +21,7 @@ func TestNewExperimentMeasurer(t *testing.T) {
 	if measurer.ExperimentName() != "web_connectivity" {
 		t.Fatal("unexpected name")
 	}
-	if measurer.ExperimentVersion() != "0.3.0" {
+	if measurer.ExperimentVersion() != "0.4.0" {
 		t.Fatal("unexpected version")
 	}
 }