Copilot CLI: Support BYOK Model Providers

[DonJayamanne]

[jimstrang]

Pushed extension settings support for BYOK configuration (github.copilot.chat.cli.provider.*) — settings take precedence over env vars, model switching works live without restart. See comment on PR #5038.

[DonJayamanne]

@jimstrang thank you for expressing interest in this and your continued effort on the PR.
However I'm unable to take a look at this PR just now, I need will take a look some time next week
As mentioned in the PR, i need to discuss this feature and figure out a plan for this, BYOK is already supported in VS Code for non-Copilot CLI chat, I want to ensure we do this in a way that works with regular VS Code as well, i.e. in a consistent manner.