Optimize "Best" Snapshot

**Describe what you are trying to accomplish and why in non technical terms**
I want to ability to disable the snapshot updates, so that the snapshot that is displayed for an event is the snapshot that triggered that event. the benefit of this would be: easy viewing of the image that triggered event in the "events" tab in the UI; and when you have an object that triggered an event that leaves all required zones but snapshot gets updated to an image of that object not the required zones it creates confusion of why that triggered an event without looking into the clip to see what happened.

**Describe the solution you'd like**
I would like a Boolean true/false property in the frigate config for snapshots (something like keep_best or keep_first) to enable either keeping the best snapshot (current functionality) or keeping the snapshot of the image that triggered the event.

The issue names no files, tests, or entry points. Start by locating the snapshot configuration and the event snapshot handling, then trace how the image that triggers an event is selected. Done means the configuration can choose between retaining the best snapshot and retaining the triggering snapshot, with the behavior covered by appropriate tests.

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