Expose custom httpsAgent for Axios

[na-wu]

Customers should be able to pass in a custom httpsAgent under httpsAgent

import { Method, Environments } from 'method-node';
import HttpsProxyAgent from 'https-proxy-agent';

const customAgent = new HttpsProxyAgent('url');

const method = new Method({
  apiKey: '<API_KEY>',
  env: Environments.dev,
  httpsAgent: customAgent,
});