[Feature Request]:Add openrouter.ai as LLM model provider

[wgong]

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Feature Request Description

to simplify mcp_agent.secrets.yaml configuration, it would be good to support openrouter.ai as LLM provider,
then one uses a single secret to access multiple models including openai/anthropic/google

I can work on this feature after becoming familiar with deepcode codebase

Additional Context

No response