Keep the first snapshot and thumbnail for an event

[Mincka]

I use the notification API to get a thumbnail in notification when an event starts, then I silently replace the notification when the event ends to update the clickaction with the clip. I want to keep a single notification for a single event.

I would like to keep the original thumbnail in my notification, but it gets replaced for various reasons (like also discussed in #4170 and #3820). I would prefer to keep the original thumbnail because the origin and the direction of the person is clearer when the event starts in my case. If someone is going and leaving in the same event, the meaning of the snapshot could be completely different in the interval.

My suggestion would be to keep and expose the first snapshot and thumbnail like this:
https://HA_URL/api/frigate/notifications//thumbnail-start.jpg
https://HA_URL/api/frigate/notifications//snapshot-start.jpg

Another usage could be to use the first image in Frigate event lists when hovering a thumbnail, so we would be able to see first and "best" thumbnail for each event. Similar idea could be used in detailed view with "First snapshot" / "Snapshot".

In the meantime, I could use a script to save the first thumbnail on disk and send it again with my updated notification.