Add multiple BYOK model capability in copilot cli

### Describe the feature or problem you'd like to solve

Currently copilot CLI supports a single BYOK model with env var. I want to enable multiple models through it which is difficult. Inside copilot TUI, I can NOT switch to another BYOK model, I have to terminate the session and set the new env vars and then reopen copilot CLI again.

### Proposed solution

Provide a json in copilot directory which copilot cli will read and use to list all the models inside.

### Example prompts or workflows

I could then do this

copilot --model

instead of resetting the env variables

### Additional context

_No response_