ToolHandle: add EnableTool tagged bool to control retrieval

The syntax of

  ToolHandle<IMyTool> m_tool;
  bool m_useToolFlag;
...
  ATH_CHECK( m_tool.retrieve( DisableTool{ !m_useToolFlag } ));

is a little confusing. This MR adds another TaggedBool to make it a little clearer:

   ATH_CHECK( m_tool.retrieve( EnableTool{ m_useToolFlag } ));

what are people's thoughts on DisableTool / EnableTool vs. DisableToolIf / EnableToolIf