phsl / new-api

- Introduced a new `FunctionResponse` type to encapsulate function call responses, improving the clarity of data handling.
- Updated the `GeminiPart` struct to include the new `FunctionResponse` field, allowing for better representation of function call results in Gemini requests.
- Modified the `CovertGemini2OpenAI` function to handle tool calls more effectively by setting the message role and appending function responses to the Gemini parts, enhancing the integration with OpenAI and Gemini systems.