dns-stats: Unbound DNS via OPNsense

### Description

With a lot of OPNsense users out there, would be great to be able to display Unbound DNS stats from OPNsense in the dns-stats widget.

OPNsense has [useful endpoints](https://docs.opnsense.org/development/api/core/unbound.html) for retrieving the stats

Some acrobatics are required to handle the structure of the response (nested structs) for the graph and top domain data.

I've been interested in contributing and have worked on something which seems to be working well for me locally. Don't have a great deal of experience with go so feedback/improvements/optimisation is welcome!