Asserting the origin of a WIS Notification Message

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Hashing & signing a WIS2 Notification Message at the point of publication would allow subscribers to assert that a message came from a given origin and has not been tampered with in transit. However, this functionality would require WIS2 Nodes to digitally sign their messages. The public key infrastructure (PKI) needed to underpin such functionality is complex and would be hard to manage. There is no clear requirement for this behaviour - yet. But we may need something like it in the future. If so, a mechanism is required to convey the digital signature with the message.