prometheus-node-exporter-lua: add modemmanager exporter

prometheus-node-exporter-lua: add modemmanager exporter

Add a Prometheus collector for ModemManager that exports cellular modem
signal metrics via mmcli. Supports multiple modems (labeled by D-Bus
object path), exports overall signal quality and detailed per-technology
signal parameters (LTE, NR5G, UMTS, GSM, CDMA, ...).

Requires signal refresh to be enabled on the modem:
  mmcli -m <id> --signal-setup=<interval_seconds>

Tested on: ath79/generic, GL.inet GL-X300B, OpenWrt 23.05.5

Co-authored-by: Claude <redacted>
Signed-off-by: Jean-Laurent Girod <redacted>