Update to allow user input OpenAI API key

[xmliszt]

We wanna give user the option to input their own API key. So we gonna provide users the following options:

• If no API key provided, by default we use Google Gemini Pro which is still currently FREE and limited to 15 RPM
• If OpenAI API key provided, store it with user's profile. (Shall we do end-to-end encryption to protect?)
  • User is able to select the model to use (GPT3.5 or GPT4)
• In the evaluation result, show a badge that this result is generated by which model.
• We need to add a disclaimer and Privacy Policy and Terms of Use.