add example which registers and uses go custom elements

this example has an assert that checks wether or nor the finalizers ran correctly. If they don't then a mem leak will happen

examples/plugins/registered_elements/internal/common/assert.go

@@ -0,0 +1,11 @@
+package common
+
+import "fmt"
+
+var FinalizersCalled int = 0
+
+func AssertFinalizersCalled(x int) {
+	if FinalizersCalled != x {
+		panic(fmt.Sprintf("finalizers did not run correctly, memory leak, wanted: %d, got: %d", x, FinalizersCalled))
+	}
+}