include/ipc: Ensure complete IPC message is signed

include/ipc: Ensure complete IPC message is signed

The current implementation does not sign fields other than the user
data in IPC messages. This makes it impossible to recognize if the
other fields have been tampered with.
This commit addresses the issue by wrapping the IPC message in an
envelope and putting the signature of the IPC message in the envelope.
This makes sure there is no unsigned data in the communication.