Fix webpack dev server when behind proxy

What does this MR do?

Explicitly sets the scheme, host, and port that WebpackProxy uses to connect to webpack-dev-server instead of simply rewriting the HTTP_HOST header. This ensures that complicated sets of headers in the request don't cause issues for the proxy. (for example HTTP_X_FORWARDED_HOST was causing trouble)

Are there points in the code the reviewer needs to double check?

Everything should still work as normal, but running webpack through GDK behind a proxy should no longer cause issues.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

/cc @DouweM