samples: ipc: Modify icmsg_me sample allowing any multiendpoint backend

There are more ipc_service backends that supports multiple endpoint.
This sample can be used for any of those backends, so this commits
makes the sample more generic. The default backend it still icmsg_me,
but now, the sample has files and instructions for icmsg_with_buf
backend.

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