feat: allow expressions for TestElement.enabled property

[vlsi]

Description

This is a proof-of-concept implementation for conditional TestElement.enabled.
The downside is that isEnabled() call would be added for every sampler, listener, pre-processor, pos-processor, and so on which might slow down the execution.

Motivation and Context

See #6006

How Has This Been Tested?

See EnabledWithVariablesTest

Screenshots (if appropriate):

WIP:

<hashTree>
  <DebugSampler guiclass="TestBeanGUI" testclass="DebugSampler" testname="Debug Sampler" enabled="true">
    <boolProp name="displayJMeterProperties">false</boolProp>
    <boolProp name="displayJMeterVariables">true</boolProp>
    <boolProp name="displaySystemProperties">false</boolProp>
  </DebugSampler>
  <hashTree/>
  <DebugSampler guiclass="TestBeanGUI" testclass="DebugSampler" testname="Debug Sampler" enabled="false">
    <boolProp name="displayJMeterProperties">false</boolProp>
    <boolProp name="displayJMeterVariables">true</boolProp>
    <boolProp name="displaySystemProperties">false</boolProp>
  </DebugSampler>
  <hashTree/>
  <DebugSampler guiclass="TestBeanGUI" testclass="DebugSampler" testname="Debug Sampler" enabled="${__javaScript(false)}">
    <boolProp name="displayJMeterProperties">false</boolProp>
    <boolProp name="displayJMeterVariables">true</boolProp>
    <boolProp name="displaySystemProperties">false</boolProp>
  </DebugSampler>
  <hashTree/>
</hashTree>

Checklist:

• UI: select the location and the control style
• UI: implement saving the plan to jmx
• UI: implement loading the plan from jmx
• Verify if TestBean-based components work as well
• Support ..RequestDefaults. The logic of merge...Element probably needs ignoring disabled elements
• Decide if interface JMeterGUIComponent needs to support expressions. Currently there's boolean getEnabled() and setEnabled(boolean)
• Add documentation
• Analyze if the following is still needed:

  jmeter/src/core/src/main/java/org/apache/jmeter/gui/tree/JMeterTreeModel.java

  Lines 169 to 175 in 91ed127

  	// This check the state of the TestElement and if returns false it
  	// disable the loaded node
  	try {
  	newNode.setEnabled(component.isEnabled());
  	} catch (Exception e) { // TODO - can this ever happen?
  	newNode.setEnabled(true);
  	}