title: JUnit 5, pass information from test class to extension
body: <p>I am trying to write an extension for Junit5 similar to what I had for Junit4 but I am failing to grasp how to do that in the new (stateless) extension system.</p>
  
  <p>The idea in the previous version was that user could pass information into extension class and hence change the way it behaved. Here is a pseudo snippet showing approximately what is used to do:</p>
  
  <pre><code>public void MyTest {
  
    // here I can define different behaviour for my extension
    @Rule MyCustomRule rule = MyCustomRule.of(Foo.class).withPackage(Bar.class.getPackage).alsoUse(Cookies.class);
  
    @Test
    public void someTest() {
      // some test code already affected by the @Rule
      // plus, user has access to that class and can use it, say, retrieve additional information
      rule.grabInfoAboutStuff();
    }
  }
  </code></pre>
  
  <p>Now, I know how to operate JUnit 5 extension, what lifecycles to use etc. But I don't know how to give the test-writer the power to modify my extension's behaviour with JUnit5. Any pointers appreciated.</p>