Bluetooth: Add management command to allow use of debug keys

Originally allowing the use of debug keys was done via the Load Link
Keys management command. However this is BR/EDR specific and to be
flexible and allow extending this to LE as well, make this an independent
command.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>