ipc_service: icmsg: Add "unbound" functionality

In some cases, CPUs that may need to reset or
temporary stop communication. This commit adds "unbound"
functionality that provides a callback to IPC service user
when connection was interrupted for some reason, e.g.
expected or unexpected CPU reset, closing the
endpoint. The "unbound" callback is optional to implement
by endpoints. This commit implements it in the ICMsg
backend.

Signed-off-by: Dominik Kilian <Dominik.Kilian@nordicsemi.no>