Improve Add Custom Model flow: make it easy to import models from Open Router, Together AI, local model, etc.

[Original title: "auto populate models from model agregators (openrouter, together,…)"]

instead of creating each model for openrouter, together, … in model selection menu, group each agregator in it's subfolder and dynamicly retrieve compatibles models (with possible filters : vison/function/context/licence…)

those agregators api keys should be hard saved in TM (has openai and claude keys are now)