feat: Add support for Fetch Implementation

[danilvalov]

Issue: #999

Usage example:

import { GoogleGenAI } from '@google/genai';
import { socksDispatcher } from 'fetch-socks';

const socks5Dispatcher =  socksDispatcher({
  type: 5 as const,
  host: 'some_proxy_ip_or_domain',
  port: 1080,
});

const fetchWithSocks = async (
  input: RequestInfo | URL,
  init?: RequestInit
): Promise<Response> => {
  return fetch(input, {
    ...init,
    dispatcher: socks5Dispatcher,
  } as RequestInit & { dispatcher: typeof socks5Dispatcher });
};

const googleGenAI = new GoogleGenAI({
  apiKey: 'your_api_key',
  fetchImplementation: fetchWithSocks,
});

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[shmishra99]

@danilvalov , Thank you for raising this PR. I have escalated it to the team for review.