Add TransactionUUID to SimpleEvent spec

[kristinapathak]

https://github.com/xmidt-org/docs/blob/main/content/docs/wrp/simple-messages.md

The transaction_uuid should be added as a required field for the Simple Event. Do we want the same description/functionality as the Request-Response?

This SHOULD be a UUID, but the web router SHALL NOT validate this data. The web router SHALL treat this data as opaque.

[schmidtw]

Yes, that is the right wording.