Add UUID as a part of SimpleEvent

[kristinapathak]

Currently, glaukos is utilizing a bug in Caduceus in order to skip over an event in the event history that is the same event sent to the webhook. The transaction UUID is meant to be no longer be set/created in Caduceus as a part of this Caduceus issue:
xmidt-org/caduceus#174 (comment)

A UUID is helpful for identifying events. It would be nice to add it as a required field to SimpleEvent in some form, even if TransactionUUID doesn't make sense.

[schmidtw]

I agree, I think excluding the TransactionUUID was a mistake in the SimpleEvent. We'll need to rev the spec to add this.

[kristinapathak]

This would implement the change in the spec described in xmidt-org/docs#74.