feat: initial implementation

[javier-godoy]

Check that com.flowingcode.vaadin.addons.example is included in the allowed-packages list. If missing, show an alert in the browser (the check is only performed for Spring-Boot applications in development mode, and it's ignored otherwise).

package com.flowingcode.vaadin.addons.example;

public class MyComponent extends Div {

  public MyComponent() {
    AllowedPackageChecker.check(this, MyComponent.class);
  }

}

[paodb]

LGTM