Add unifi unpoller

[lukegiuliani]

Lots of hobbyists have Unifi gear for their home networking. This PR adds in the Unpoller exporter. It follows the same conventions as upstream (default off to reduce bloat, separate docker-compose etc).

Note that for prometheus to be able to pull from it it does need to be on the same network for dockers dns to resolve, hence the addition of:

networks:
  back-tier:
    name: internet-monitoring-back-tier

in templates/unpoller-docker-compose.yml.

I think this change would also fix the issue in upstream where explicit IPs are used for shelly plug and airgradient, allowing them to be referenced by container name instead.

glhf

LG