feat: Add support for per-request metadata/headers in GenerativeModel.generate_content.

[mukeshgupta7]

Description of the change

Modified the GenerativeModel.generate_content and GenerativeModel.generate_content_async method to accept an additional parameter extra_headers that would be passed to the underlying client method. This would allow users to set headers on a per-request basis while maintaining backward compatibility.

Added the required tests in test_generative_model.py and test_generative_model_async.py

Type of change

Feature request

Checklist

• [ ] I have performed a self-review of my code.
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• [ ] I have verified that my change does not break existing code.
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