include/ipc: Add module for messaging-based IPC

include/ipc: Add module for messaging-based IPC

This commit adds the ipc module, which implements a messaging-based
approach to IPC using JSON objects that are transmitted via queues.
The current implementation authenticates messages using GPG, which
allows simple GPG-based authorization schemes to be implemented on
top of this mechanism.
This commit includes a JSON Schema for toolbox IPC messages and a
Python script for validation, both of which will be integrated into
the test suite.