createEnvelope abstract method
Creates a custom message envelope with specified routing metadata.
Builds an Envelope from explicit sender/receiver/timestamp parameters, forming the header of a message (routing information).
sender is the ID of the message sender.
receiver is the ID of the message receiver (user/group).
time is the message timestamp (defaults to current time if null).
Returns a custom Envelope instance with routing metadata.
Implementation
Envelope createEnvelope({required ID sender, required ID receiver, DateTime? time});