Update WRP simple messages spec

https://github.com/xmidt-org/docs/blob/main/content/docs/wrp/simple-messages.md

The device ID isn't always the simple event's source anymore, so we should update the spec to specify the source to be the creator of the event and add a separate `device_id` field.  The `device_id` field should also be added to the RequestResponse WRP, so that it can be specified in responses.