Add Idempotency Key to the pleroma:chat_update streaming event payload
For optimistic / nonblocking posting in chat, it would be useful to have the Idempotency Key (or something similar, e.g.,
client_message_id) in the
pleroma:chat_update event payload.
The reason is that the streaming event arrives earlier than the
POST /api/v1/pleroma/chats/:id/messages response, and without the idempotency key (or a similar identifier) it's difficult to tell for sure if the event belongs to the message that FE has just posted or it should be handled as a different message.