Support HA addon_config mount point

[Eskander]

Home Assistant added a dedicated addon_config mount point for add-ons a while ago.

This allows HA to backup add-on config files with the correct version before updates, and the folder still mounts to /config inside the container so no change from Frigate's side is necessary.

Since 0.14.0 is already a major update with backwards incompatible frigate.db, it may be a good opportunity to adopt and benefit from this change.