feat: Ability to modify headers and custom fetch implementation

[shahradelahi]

This PR introduces a new feature that allows users to modify fetch headers or use a custom fetch implementation. It uses nodejs/undici for making requests. According to the Node.js development team, the Undici module has better performance and compatibility.

Examples of Usages

Use an HTTP Proxy

Using a proxy is useful for those who are behind a corporate firewall and trying to reach out to the resend API server. Here is an implementation for custom fetch for using HTTPS proxy:

import { Resend } from 'resend';
import { fetch, ProxyAgent } from 'undici';
import 'dotenv/config';

if (!process.env.RESEND_API_KEY) {
  throw new Error('RESEND_API_KEY env is not defined');
}

const resend = new Resend(process.env.RESEND_API_KEY, {
  fetch: (input, init) => {
    return fetch(input, {
      ...init,
      dispatcher: new ProxyAgent('http://localhost:8080')
    });
  }
});

export default resend;

Please review this pull request and let me know if you have any feedback. Thank you!