[JENKINS-52189] Ensure GraphListeners are notified of the FlowStartNode too

[svanoort]

JENKINS-52189

We might consider initializing the CpsThreadGroup before the FlowStartNode (in CpsFlowExecution) to make this more "correct" but I'm a bit concerned that might have unintended consequences.

[svanoort]

I'm leaving this in, in case we do get a proper CpsThreadGroup down the road, so it does The Right Thing (tm).

[t-8ch]

This will currently not work for GraphListeners attached using FlowExecution.addListener() in FlowExecutionListener.onRunning().
The FlowStartNode is emitted before FlowExecutionListener.onRunning().

It is obviously possible to use a global extension, but I was under the impression that onRunning() was an intended place to add new listeners.

[jglick]

Note: might solve a minor display issue in logs that I noticed in JEP-210 (see #252), that we display the FlowEndNode but not the FlowStartNode. Depends on the timing of this event I suppose.