Add support for environment variables as authentication alternatives to
OAuth flow for Copilot. Closes #31172

We can include the token in HTTPS request headers to hopefully resolve
the rate limiting issue in #9483. This change will be part of a separate
PR.

Release Notes:

- Added support for manually providing an OAuth token for GitHub Copilot
Chat by assigning the GH_COPILOT_TOKEN environment variable

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Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>