Http proxy for fetch

[zicklag]

Changes

• Adds support for using fetch behind an HTTP proxy.
• Users can now configure the http proxy through the HTTP_PROXY, HTTPS_PROXY, and NO_PROXY environment variables.

Testing

There are no additional tests. We could potentially add tests, but it would require standing up an HTTP proxy to test through during the test, which might be non-trivial.

It would be up to your discernment whether or not we need to pursue that.

Docs

We should add a section to the docs about use behind HTTP proxies. Especially because most things that support HTTP proxies will read both the uppercase environment variables, and the lowercase environment variables, but the global-agent package I used to support proxies here only supports the uppercase environment variables, so that could be a gotcha, for some people. ( It almost was for me. )