mgmt/osdp: Rework secure channel key management

OSDP command KEYSET is used to set the secure channel base key for all
connected PDs. This key is then used to derive the session keys for each
secure channel session. When the app wants to set the this key, it has
to issue a command and then both the CP/PD has to be notified of this
change so they can store this key in a non-volatile medium for future
operations across power cycles.

The current implementation of OSDP had limited support for key
management. This patch adds all the bells and whistles needed to handle
keyset command/event in the CP/PD application.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>