Feature request: show v3 message sequence id in push server + lookup endpoint

[nmalzieu]

Is your feature request related to a problem?

V3 Messages are bigger than v2. In Converse's push notification server, something as simple as a reply can trigger a "payload too big" to send through FCM or APNS.

Describe the solution to the problem

By making the v3 message sequence id visible on the network along with the envelope, if the payload is too big, the push server can just send the v3 message sequence id to Converse (or any other app)
Then, the app can query the network with that message sequence id to get back the full envelope and decode it to display the notification (this endpoint does not exist yet)

Describe the uses cases for the feature

No response

Additional details

No response