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.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.