refactor(engine): allow scripts to register experiments (#860)

See https://github.com/ooni/probe/issues/2216

internal/registry/allexperiments.go

@@ -0,0 +1,12 @@
+package registry
+
+// Where we register all the available experiments.
+var allexperiments = map[string]*Factory{}
+
+// ExperimentNames returns the name of all experiments
+func ExperimentNames() (names []string) {
+	for key := range allexperiments {
+		names = append(names, key)
+	}
+	return
+}