mgmt/osdp: Add support for event delivery and notifications

The CP app sends PD a "command" and the PD responds to it. Some times,
the PD has something that it wants to tell the PD which it does so in
response to POLL command. Both CP and PD apps need a way to exchange
these info over the OSDP bus. To archive this we will introduce what are
called "events" that allow the PD app to enqueue and CP app to get
notified.

This is analogous to the incumbent "commands" abstraction where, the CP
app enqueues a command and the PD app gets notified of it.

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