add proxy support

[chiefjester]

• improved @germanftorres PR by enabling https urls
• proxy: remove secure option to enable ssl
• add changeOrigin true to make proxy work
• Add support for websockets in proxy build script
• fixes Improve proxy support with http-proxy #371

[chiefjester]

@FredKSchott @germanftorres
So some improvements I made. I had SSL disabled and changeorigin to true. This would make https proxy work normally. The alternative is add our own certificate in snowpack which I don't think is a good idea since that's another thing we need to maintain.

This would now work on both POST and GET methods, as mentioned here

This will also fix @fcamblor issue as I've tested, it forwards the encoding headers correctly:

Some room for improvements, @FredKSchott do you think we need to pass the options for the the http-proxy in order for other people to extend it?

As it currently stands, if someone wants to extend it, they can just make a new server and just pass it in the proxy and they can do whatever they want with that http-server. So I personally don't think we need to make it more complicated as it is.

[chiefjester]

@FredKSchott 👋 rebased more context here

[FredKSchott]

This makes sense! Thanks for explaining your reasoning. We'll probably want to move this into the createProxy call when we add support for user configuration, but I think this is fine for now.