Allow users to select the base model for requests.
Starting off with the following (corresponding model ID for openrouter)

• Claude 3.5 Sonnet anthropic/claude-3.5-sonnet
• Gemini Flash 2.0 google/gemini-2.0-flash-001
• DeepSeek V3 deepseek/deepseek-chat
• GPT-4o gpt-4o