Add support for no_proxy and case insensitive env variables (#9372)

### Changes proposed in this PR

- Add support for the `no_proxy` and `NO_PROXY` environment variables
  - Internally rely on urllib's [`proxy_bypass_environment`](https://github.com/python/cpython/blob/bdb941be423bde8b02a5695ccf51c303d6204bed/Lib/urllib/request.py#L2519)
- Extract env variables using urllib's `getproxies`/[`getproxies_environment`](https://github.com/python/cpython/blob/bdb941be423bde8b02a5695ccf51c303d6204bed/Lib/urllib/request.py#L2488

) which supports lowercase + uppercase, preferring lowercase, except for `HTTP_PROXY` in a CGI environment

This does contain behaviour changes for consumers so making sure these are called out:
- `no_proxy`/`NO_PROXY` is now respected
- lowercase `https_proxy` is now allowed and taken over `HTTPS_PROXY`

Related to #9306 which also uses `ProxyAgent`

Signed-off-by: Timothy Leung <tim95@hotmail.co.uk>