Added EnvHttpProxyAgent to support HTTP_PROXY by default

[10xLaCroixDrinker]

This relates to...

Closes #1650

Changes

Added EnvHttpProxyAgent and made it the default global dispatcher. The addition to index-fetch.js was required to keep those tests passing after changing the global dispatcher to a ProxyAgent

Features

Automatically detect/support HTTP_PROXY, HTTPS_PROXY, & NO_PROXY

Bug Fixes

N/A

Breaking Changes and Deprecations

Using these env vars by default could result in unexpected behavior

Status

• [x] I have read and agreed to the Developer's Certificate of Origin
• [x] Tested
• [S] Benchmarked (optional)
• [S] Documented
• [ ] Review ready
• [ ] In review
• [ ] Merge ready