feat: allow setting session_id in ctx.send

[jrriehl]

Allow users to overwrite the session id by setting it in ctx.send(). This means that an agent can send messages for a previous session and not necessarily the one given by the incoming envelope.

This can be useful for example when information needs to be collected from other agents before responding to a request.

[Archento]

Code wise this looks good but as @Dacksus already posted I also think that we should discuss before adding the session_id to messages as this would potentially allow session spoofing if implemented incorrectly.
Maybe we should go forward with the Dialogue implementation first and create a good working example from that.

[jrriehl]

Agreed, closing this.