Add latest frigate version to stats endpoints

[NickM-27]

This is in order to support the efforts of blakeblackshear/frigate-hass-integration#235, frigate will check github for the latest version tag once on startup so that services (and potentially frigate itself in the future) can be aware if there is an update.

I found that the stats_tracking was the "cleanest" way I could think of to pull the value only once and have it be included in both http and mqtt calls. If there is a better solution happy to reorganize this 👍

[drinfernoo]

What if Frigate isn't restarted for an indeterminate amount of time? It will only check for updates whenever it is?

[NickM-27]

What if Frigate isn't restarted for an indeterminate amount of time? It will only check for updates whenever it is?

I spent some time thinking about it, but I would rather not spend time / resources polling since this should only consider full releases and not betas which means there would be a lot of redundant checks relative to the number of checks that found a new version. And since it is just supporting a HASS entity and nothing too important.

If this is deemed more important than I think it is, there's a few ways I can think to have it poll once a day.