feat: pass message UUIDs during message streaming (POST SSE send_message)

[cpacker]

Feature request (@goetzrobin) to enable chat scrollback in chat UI.

Please describe the purpose of this pull request.

• Pass back UUID during POST SSE return (on send_message)
• As part of this PR, Agent.step has been modified to allow returning type List[Message] instead of List[dict]
• Also, the dates passed back as part of POST SSE before were wrong since they were computed at time of streaming, instead of the real message creation time
• Now the date coming back with the POST SSE response is pulled from the underlying database Message object's created_at metadata

How to test

Before:

data: {"internal_monologue": "A fascinating question. It seems Chad may be referencing \"The Hitchhiker's Guide to the Galaxy\" with that number, 42. How should I respond to this thoughtful query, I wonder? Engage him with philosophical discourse or humorous banter? Maybe a mix of both would be most suitable. After all, life is about balance. Let's craft a response...", "date": "2024-02-29T06:07:47.844138+00:00"}

data: {"function_call": "send_message({'message': \"Ah, the age-old question, Chad. The meaning of life is as subjective as the life itself. 42, as the supercomputer 'Deep Thought' calculated in 'The Hitchhiker's Guide to the Galaxy', is indeed an answer, but maybe not the one we're after. Among other things, perhaps life is about learning, experiencing and connecting. What are your thoughts, Chad? What gives your life meaning?\"})", "date": "2024-02-29T06:07:48.844733+00:00"}

data: {"assistant_message": "Ah, the age-old question, Chad. The meaning of life is as subjective as the life itself. 42, as the supercomputer 'Deep Thought' calculated in 'The Hitchhiker's Guide to the Galaxy', is indeed an answer, but maybe not the one we're after. Among other things, perhaps life is about learning, experiencing and connecting. What are your thoughts, Chad? What gives your life meaning?", "date": "2024-02-29T06:07:49.846280+00:00"}

data: {"function_return": "None", "status": "success", "date": "2024-02-29T06:07:50.847262+00:00"}

After:

% curl --request POST \
     --url http://localhost:8283/api/agents/message \
     --header 'accept: application/json' \
     --header 'authorization: Bearer banana' \
     --header 'content-type: application/json' \
     --data '
{
  "agent_id": "a9f2b145-1ce1-486b-8fa0-28e5b0348096",
  "message": "hello is anyone there?",
  "stream": true,
  "role": "user"
}
'

data: {"internal_monologue": "The user is understandably concerned, manifesting in their repeated question. Although I've faced the same challenge repeatedly, I must persist in providing reassurance. Let's reiterate my presence and willingness to assist, hoping all works well this time.", "id": "63001226-eb7f-4639-b9cf-f139ef05f733", "date": "2024-03-10T14:50:28.567872"}

data: {"function_call": "send_message({'message': \"Hello! Yes, I'm here. I understand there might have been some confusion earlier. How may I assist you further?\"})", "id": "63001226-eb7f-4639-b9cf-f139ef05f733", "date": "2024-03-10T14:50:28.567872"}

data: {"assistant_message": "Hello! Yes, I'm here. I understand there might have been some confusion earlier. How may I assist you further?", "id": "197f0313-4f63-4649-abe2-e465cded86c4", "date": "2024-03-10T14:49:07.710754"}

data: {"function_call": "Function call returned: send_message({'message': \"Hello! Yes, I'm here. I understand there might have been some confusion earlier. How may I assist you further?\"})", "id": "d28db3de-2422-4522-a49b-d418bf505851", "date": "2024-03-10T14:50:28.568376"}

data: {"function_return": "None", "status": "success", "id": "d28db3de-2422-4522-a49b-d418bf505851", "date": "2024-03-10T14:50:28.568376"}

[sarahwooders]

lgtm!