Add proxy support for the editor

[timothyqiu]

Follow-up of #47257.

• Adds proxy support for HTTPRequest.
• Adds network/http_proxy/{host,port} editor settings.
  • Labeled as "HTTP Proxy" and it's meant to be used for both HTTP and HTTPS requests. This is the same convention as seen in Android Studio's proxy settings.
• Makes Asset Library and Export Template Manager setup proxy according to the editor settings.

The proxy has to be set manually in Editor Settings. It would be a separate PR trying to figure out default proxy from environment variables on different platforms 😜

Fixes #6100.

[timothyqiu]

I can do a separate 3.x PR for these two proxy PRs after this one is merged. Adding the cherrypick: 3.x tag just as a reminder 😄

[Calinou]

The proxy has to be set manually in Editor Settings. It would be a separate PR trying to figure out default proxy from environment variables on different platforms stuck_out_tongue_winking_eye

On desktop platforms, I believe the HTTP_PROXY and HTTPS_PROXY environment variables are de facto standards nowadays. You will likely have to check for both uppercase and lowercase variants though, since environment variable names are case-sensitive on macOS and Linux. (It seems that a lot of software configures https_proxy as lowercase.)

[timothyqiu]

Yeah, these environment variables are definitely the way to go. Meanwhile, on Windows and macOS, desktop applications tend to honor the system proxy settings which have to be queried via API, I think.

p.s. Personally, I use the lower cased https_proxy a lot since it's easier to type 🤣

[mhilbrunner]

This is a straighforward change after the proxy PR. Looks good, and thanks for working on this!
I'll let @Faless merge.

[Faless]

Thanks!

[almusx]

Authenticated proxy doesnt work.

[Calinou]

Authenticated proxy doesnt work.

Please open a new issue for this, while detailing your OS and proxy configuration (which authentication method does it use?).

• Edit: Issue opened: Add support for authenticated HTTP proxy in HTTPClient #76442