Review Jetty XML syntax to allow calling JDK methods

[sbordet]

Jetty version(s)
10+

Description
Jetty XML syntax allow to call methods of any class via reflection.

However, when calling JDK methods, they may return JDK-private implementation classes that are not accessible from Jetty code.
This surfaced in particular while working on #8863, where a new Jetty module for the thread pool that supports virtual threads need to call JDK methods that return JDK-private classes.

For that example, we have:

<!-- returns JDK private class ThreadBuilders.VirtualThreadBuilder -->
<Call class="java.lang.Thread" name="ofVirtual">
  <!-- Throws IllegalAccessException: class org.eclipse.jetty.xml.XmlConfiguration$JettyXmlConfiguration 
cannot access a member of class java.lang.ThreadBuilders$VirtualThreadBuilder (in module java.base) 
with modifiers "public volatile" -->
  <Call name="name">virtual</Call>
</Call>

The JDK-private class ThreadBuilders.VirtualThreadBuilder is not accessible, but its JDK-public ancestor Thread.Builder is.

The proposal is to change slightly the semantic of the Call element to support this:

<!-- static invocation -->
<Call class="java.lang.Thread" name="ofVirtual">
  <!-- virtual invocation -->
  <Call class="java.lang.Thread$Builder" name="name">virtual</Call>
</Call>

The second Call element specifies the class attribute that should be used to reflect the method named name but it performs a virtual call, not a static call (because it is in the context of an enclosing Call element).

Ambiguities are easily resolved because it's always possible to ask a Method if it is static and if so perform the invocation passing null as the target of the invocation.