Add Tetrate Agent Router Service (TARS) as provider

[arifsetiawan]

Tetrate Agent Router Service (TARS) recently launched as a new AI router similar to OpenRouter. It provides unified access to multiple AI providers, with features like automatic fallbacks, and usage monitoring configured through its web UI. Learn more at https://tetrate.io/blog/announcing-tetrate-agent-router-service.

We would like to add TARS as a native provider option in Kilo Code, similar to how other AI routers are supported. This would Eliminate manual base URL configuration - just select "TARS" and enter API key and allow users to directly use the model without need to fill model details.

I'm willing to contribute and maintain the implementation. This will follow existing provider implementation.