feat(gemini): add support for gemini-exp-1114 model
- Add gemini-exp-1114 to ModelList in constant.go
- Update GetRequestURL in adaptor.go to use v1beta API version for gemini-exp-1114
- Keep backward compatibility for other models
This change enables the use of the experimental gemini-exp-1114 model by correctly routing its requests to the v1beta API endpoint while maintaining existing functionality for other models.
