Tracking: RD control

[chrysn]

Both for experimentation and operation we'll need to control behavior of a node; exemplary controls are:

• Switch to FT mode (and back, possibly with a grace period to encourage mobility procedures)
• Gather peer nodes, their stats and RSSI-2
• Trigger and gather debug scans and RSSI measurements

The early demos in diondokter/nrf-modem#52 do this by hardcoding and defmt; Nordic's dect_shell does this through a shell; my preferred approach is a CoAP server (as that will also allow secure remote access easily), possibly aided by the Jelly shell.