Feature/Add support for per-request metadata/headers

[drona-gyawali]

Description of the change

PR introduces support for setting per-request metadata/headers in GenerativeModel.generate_content. Previously, headers could only be set at the client configuration level using default_metadata, making it difficult to specify user-specific headers dynamically for each request. This enhancement improves flexibility, particularly for integrations with proxy services like Helicone that require user-specific headers.

Technical

Added a new parameter extra_headers: dict[str, str] | None .

Ensured that extra_headers are properly merged with existing request metadata.

Preserved backward compatibility by making extra_headers optional.

Updated function documentation to reflect the new parameter.

closes #698

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[drona-gyawali]

@chitalian, I manually checked this feature and it works. As of now, I haven't implemented any test cases because I want you to review it first. If the feature makes sense and receives your approval, I will create test cases with your permission. Thank you!

[chitalian]

Looks good @drona-gyawali ! Thanks for making this fix