Support https behind proxy

[AlexandreBonaventure]

Is your feature request related to a problem? Please describe.
We'd like to run vite with a docker development setup. In this case, the https component is already handled by our local proxy (like this one https://github.com/codekitchen/dinghy-http-proxy).
We can't provide any certificates (because certificates are handled top-level in the proxy) https://github.com/vitejs/vite/blob/master/src/node/server/index.ts#L143

Currently:
If we disable https on vite. This is causing different issues with the websocket connection: Mixed content for the websocket connection (because without https vite is serving the connection with ws:// protocol) see screenshot down below

We use to do that a lot with vue-cli where it was working fine, because webpack-dev-server is handling wss connection through a endpoint routed by Express (same port as the main app) as opposed to be accessible at another port.

[linchen1987]

I encountered the same problem. I want to run vite in https://gitpod.io