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

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

internal/registry/example.go

@@ -0,0 +1,28 @@
+package registry
+
+//
+// Registers the `example' experiment.
+//
+
+import (
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/example"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+func init() {
+	allexperiments["example"] = &Factory{
+		build: func(config interface{}) model.ExperimentMeasurer {
+			return example.NewExperimentMeasurer(
+				*config.(*example.Config), "example",
+			)
+		},
+		config: &example.Config{
+			Message:   "Good day from the example experiment!",
+			SleepTime: int64(time.Second),
+		},
+		interruptible: true,
+		inputPolicy:   model.InputNone,
+	}
+}