feat: ChatGPT import carries over timestamps

[aniqueta]

Is your feature request related to a problem? Please describe.
Open-WebUI does an excellent job of importing ChatGPT exported JSON. However, it does not carry over timestamps for either the conversation or the messages in the conversation. Open-WebUI can do this for Ollama/Open-WebUI exported JSON though.

Describe the solution you'd like
Open-WebUI does imports the timestamps for ChatGPT exported JSON conversations and messages.

Describe alternatives you've considered
I've tried to see if I can manually edit the ChatGPT exported JSON to make this work, with the hopes of writing a script. It appears the format is quite different, and reading the #498 pull request, it seems a separate logic altogether is used for ChatGPT import.

Thanks!