sysutils/nut_exporter: new port

Add a port for https://github.com/DRuggeri/nut_exporter which is a
Prometheus (net-mgmt/prometheus2) exporter for the "Network UPS tools"
(sysutils/nut).

PR: 267485
Approved by: pkubaj (mentor)
Differential Revision: https://reviews.freebsd.org/D37883