CoPilot BYOK - Add Support OpenAl Compatible

[schaveyt]

With the new ability to bring your own key to copilot, it finally opens the ability to also...specfiy a generic "OpenAI Compatible" provider.

See Cline or RooCode AI Assistant Extensions.

For that provider, the tool must accept

1. Base URL
2. Access Key (i.e password)
3. Model name (text)
4. Max Window Size (integer, optional)

this allows folks to MANY models like SambaNova, Cerebras, etc who provide OpenAI Compatible endpoints

In addition, many enterprise use LiteLLM as a proxy in front of AWS or others solutions...and it too is OpenAI Compatible

Can add a "User Warning: CoPilot using advanced prompting and tools and may not work as expected for less capable models