ipc: multi-endpoint icmsg cleanup

The multi-endpoint backend of the ipc_service subsystem contains two
roles with separated implementations: initiator and follower. There
was many code duplications for both roles. This patch introduces a new
IPC library: icmsg_me containing common code extracted from both roles
and encapsulating access to common data fields.

Signed-off-by: Hubert Miś <hubert.mis@nordicsemi.no>