Support `gemini` provider type in Bring Your Own Model (BYOM) feature

[doggy8088]

Describe the feature or problem you'd like to solve

Support for COPILOT_PROVIDER_TYPE=gemini will be awesome

Proposed solution

Support for COPILOT_PROVIDER_TYPE=gemini

Example prompts or workflows

Add support for COPILOT_PROVIDER_TYPE=gemini.

Additional context

No response