ipc: separate icmsg core from ipc_service backend

icmsg library was implemented as a ipc_service backend. It is a simple
library with minimal feature set. To preserve its simplicity while
adding more features it is useful to introduce separated ipc_service
backends with added features. To reuse most of the icmsg code for the
simplest backend and other including more features, the core of icmsg
is separated from the simplest ipc_service to an icmsg module which
can be used by multiple backends.

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