[Feature Request] Add proxy.

[Abab-bk]

I need use VPN to download godot from github and TuxFamily.
But, godots cant pass proxy by default.
So i gotta enable tun mode.
My request: Add a setting on option.

[EthanGrahn]

This should be as easy as using set_http(s)_proxy in the custom HTTPClient code here.

godots/src/http_client.gd

Line 5 in c152061

var http_request = HTTPRequest.new()

Of course options will also need to be added to the settings and they will need to be retrieved before setting the proxy.