Add tests for Gemini via openai package

[jackmpcollins]

• Add openai_gemini marker.
• Add GEMINI_API_KEY to .env.template.
• Test gemini via openai
• Add section to configuration docs on Gemini via OpenAI

Fixes #432
Related to #169 #168

---

Update: the below error appears to be resolved now.

Currently getting the following error for the parallel_tool_calls argument.

openai.BadRequestError: Error code: 400 - [
    {
        'error': {
            'code': 400,
            'message': 'Invalid JSON payload received. Unknown name "parallel_tool_calls": Cannot find field.',
            'status': 'INVALID_ARGUMENT',
            'details': [
                {
                    '@type': 'type.googleapis.com/google.rpc.BadRequest',
                    'fieldViolations': [
                        {
                            'description': 'Invalid JSON payload received. Unknown name "parallel_tool_calls": Cannot find field.'
                        }
                    ]
                }
            ]
        }
    }
]

Posted on this related google issue https://discuss.ai.google.dev/t/invalid-argument-error-using-openai-compatible/51788/9?u=jackmpcollins