include/ipc: Add JSON Schema-based data validation

include/ipc: Add JSON Schema-based data validation

When an application receives a message from `ipc_endpoint_recv()',
it has to validate the contents of the message. It would be more
convenient if the validation could be performed by the ipc module,
so that the `ipc_endpoint_recv()' function only returns valid
messages to the caller.

This commit modifies the ipc module so that the user may set a
JSON Schema that is used by the ipc module to validate the data
of incoming messages. When a schema has been set, only messages
that passed validation will be returned by `ipc_endpoint_recv()'.