Uptime sensor should be a timestamp

[mateuszdrab]

The recently added Frigate Uptime sensor in #820 is a great idea; however, it should be a timestamp similarly to the Uptime sensor provided by Home Assistant.

Reasoning is that an ever increasing counter clutters the database unnecessarily and a timestamp can be easily used to calculate the uptime in seconds or minutes using a template if needed.

It is also hard to make sense of the uptime in seconds. When providing a timestamp, HA can provide a relative calculation automatically.