Bluetooth: Add support for Path Loss Monitoring feature

This commit adds host support for the Path Loss Monitoring
feature see Bluetooth Core specification, Version 5.4,
Vol 6, Part B, Section 4.6.32.

Limited logic is required, just adding a wrapper around the
HCI command and callback for HCI event.

Add new zone - BT_CONN_LE_PATH_LOSS_ZONE_UNAVAILABLE, to
convert 0xFF path loss to a useful zone.

Add new Kconfigs and functionality to the bt shell.

Signed-off-by: Sean Madigan <sean.madigan@nordicsemi.no>