Configuration of a send message request.

Message is one unit of communication between client and server. It can be associated with a context and/or a task. For server messages, context_id must be provided, and task_id only if a task was created. For client messages, both fields are optional, with the caveat that if both are provided, they have to match (the context_id has to be the one that is set on the task). If only task_id is provided, the server will infer context_id from it.