Add support for per-request metadata/headers in GenerativeModel.gener…

[aaakash06]

re: #698

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