spec/ipc: Rename base message schema to ipc_message / ipc.message.json

spec/ipc: Rename base message schema to ipc_message / ipc.message.json

IPC base messages are stored in the "message" field of an envelope,
and the schema for the latter is called ipc.envelope.json (and the
file is called ipc_envelope.schema.json). In keeping with that
convention, the schema for IPC base messages should be called
ipc.message.json (filename ipc_message.schema.json).
This commit makes sure the naming conventions are adhered to.