For OpenRouter, add the option to use their ":nitro" variants

OpenRouter announced on Twitter that adding ":nitro" to any model name would make OpenRouter select the fastest available provider.

I think this proves helpful for audio transcription & formatting, most of the time, you want the fastest provider there is. :)

Post for reference: https://x.com/OpenRouterAI/status/2019776391143641597?s=20