Aids to desync event troubleshooting

[jorge-gonzalez-lexia]

Is your feature request related to a problem? Please describe.
Would appreciate guidance on how to troubleshoot when a desync event is detected (either in synctest mode or via GgrsEvent::DesyncDetected). Currently we only get the checksums and the remote frame where they failed. Then what?

Describe the solution you'd like
Seems like the when debugging, one would want to see the snapshots for the local and remote frames. I asked about this in the GGRO discord channel and that seemed to be the consensus. However, we did not know how to get at the relevant snapshots. I've tried getting my head around the code and it seems:

• GgrsSnapshots does not keep enough snapshots. It drops once they are confirmed which happens before desync detection (even with an interval of 1)
• synctest sessions do not have the GgrsEvent::DesyncDetected (or any events)

If we get around the above issues, then a system fn could at least log the snapshots.

Unless there's a better way to debug desync issues?

Describe alternatives you've considered
I am very new to all this, so perhaps this is a nonissue or there's a simpler way.

Additional context
See the Discord GGRO development channel thread from Dec 16