Add Structured Messages to AgentChat

[husseinmozannar]

Add ability to send structured messages between agents. Maybe define a new message type, or just extra field to the base messages.

While we're doing this you need it would be good to add voicemessage, videomessage, message with files, vectormessage..., all of these are necessary to enable broader use cases in agentchat. If the myriad of types is too much, then resort to a single message type.

[gagb]

🫡

[husseinmozannar]

@ekzhu we've discussed this previously, hope to see this in agentchat, see discussion in #5372