Gemini 2.0 Flash Thinking Mode

gemini-2.0-flash-thinking-exp is supported!

Ref: https://ai.google.dev/gemini-api/docs/thinking-mode

Other improvements:

• Support frequency_penalty, presence_penalty and max_completion_tokens
• Improve response_format handling, support "json_schema"
• Support responseMimeType: "text/x.enum"
• Misc. important fixes