google-genai: support of adding custom headers on ChatGoogleGenerativeAI

[mingshun]

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Feature request

support of adding custom headers on ChatGoogleGenerativeAI

Motivation

My code invokes google genai api by langchain is behind a proxy. I want to add custom headers to perform some data collection on the proxy.

Proposal (If applicable)

The version of @google/generative-ai which @langchain/google-genai depends on is too old. Starting from 0.11.0, @google/generative-ai adds customHeaders field to RequestOptions inteface. Maybe it can be used by ChatGoogleGenerativeAI to support adding custom headers.