added system prompt for openai

[Leo-LiHao]

What does this PR do?

Add system prompt for OpenAI Representation model, see Issue #2146

Before submitting

• [ ] This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• [x] Did you read the contributor guideline?
• [ ] Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes (if applicable)?
• [ ] Did you write any new necessary tests?