Need ability to pass chat history from the API Client

[DykiSA]

Firstly, thanks for the great hard work.

I have tried this and it is working fine for small chatting mechanism.

But when I want to use it with agents, it doesn't work since agent may send many contents in the user message.

To achieve this, it still needs many changes or maybe flow refactoring for some reasons:

1. The request handler only allowing text as a user message content
2. Only last user message is processed, any previous items in the messages except type=system are ignored.
3. Enforcing to build messages based on server history, any system message and last user message upon sending to the model provider.

I am not experienced in python, so I wish it is possible to implement this. Thanks.