feat: implement proxy handling with urllib3

Since refactoring in !64, tokens are generated using 'urllib3' instead
of the 'requests' library.

However urllib3 does not honor proxy related environement variables which
breaks pipelines using Google Cloud authentication in runners behind a
proxy.

This commit implements proxy handling with urllib3:

- Check for https_proxy, HTTPS_PROXY, http_proxy or HTTP_PROXY
  environment variables (by order of precedence),
- parse proxy url from first detected variable,
- set authentication info if present,
- use urllib3.ProxyManager() if proxy is set,
- fallback to urllib3.PoolManager() otherwise.

fixes: 5d44387979f888ed2091b68429b2357c2f4025e0