Improve the ability to add custom extensions for the containers

package org.jboss.arquillian.spock;
public class ArquillianSputnik extends Sputnik {
  import org.jboss.arquillian.test.spi.TestRunnerAdaptor;
  import org.jboss.arquillian.test.spi.TestRunnerAdaptorBuilder;
  // Methods and fields omitted for brevity, but you can see that the
  // proper packages are imported
  public void run(RunNotifier notifier) {
    // Code omitted for brevity
    final TestRunnerAdaptor adaptor = TestRunnerAdaptorBuilder.build();
    // More code omitted for brevity
  }
}

package org.jboss.arquillian.test.spi;
public class TestRunnerAdaptorBuilder {
  private static final String DEFAULT_EXTENSION_CLASS = 
    "org.jboss.arquillian.core.impl.loadable.LoadableExtensionLoader";
  private static final String TEST_RUNNER_IMPL_CLASS =
    "org.jboss.arquillian.test.impl.EventTestRunnerAdaptor";
  public static TestRunnerAdaptor build() {
    // omitted lines for brevity
    ManagerBuilder builder = ManagerBuilder.from()
        .extension(SecurityActions.loadClass(DEFAULT_EXTENSION_CLASS));
    return SecurityActions.newInstance(
        TEST_RUNNER_IMPL_CLASS, 
        new Class<?>[] {ManagerBuilder.class}, 
        new Object[] {builder}, 
        TestRunnerAdaptor.class);     
  }
}

package org.jboss.arquillian.test.spi;
final class SecurityActions {
  static <T> T newInstance(final String className,
        final Class<?>[] argumentTypes,
        final Object[] arguments,
        final Class<T> expectedType) {
    @SuppressWarnings("unchecked")
    Class<T> implClass = (Class<T>) loadClass(className);
    if (!expectedType.isAssignableFrom(implClass)) {
      throw new RuntimeException("Loaded class " + 
          className + 
          " is not of expected type " +
          expectedType);
    }
    return newInstance(implClass, argumentTypes, arguments);
  }
}