dev-server: pick up scheme and host from forwarding proxy.

[LeviticusMB]

Read and use X-Forwarded-Host and X-Forwarded-Proto when constructing the dev-server client URL.

This makes it possible to put the dev-server behind and HTTP/2 or TLS proxy, which can redirect requests not only to the dev-server but also all kinds of backend API-services.

For instance, the following nginx config can be used to serve both the dev-server and a couple of API endpoints over HTTP/2.

http {
    ssl_certificate     fullchain.pem;
    ssl_certificate_key privkey.pem;
    ssl_dhparam         ffdhe2048.txt;
    ssl_protocols       TLSv1.2 TLSv1.3;
    ssl_ciphers         ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;

    map $http_upgrade $connection_upgrade {
        default upgrade;
        ''      close;
    }

    proxy_http_version  1.1;
    proxy_set_header    Upgrade             $http_upgrade;
    proxy_set_header    Connection          $connection_upgrade;
    proxy_set_header    Host                $http_host;
    proxy_set_header	X-Forwarded-For	    $remote_addr;
    proxy_set_header	X-Forwarded-Host    $http_host;
    proxy_set_header	X-Forwarded-Proto   $scheme;

    server {
        listen          443 ssl http2;
        listen          [::]:443 ssl http2;

        location / {
            proxy_pass  http://localhost:3333;
        }

        location ~ /(my|api|endpoints).* {
            proxy_pass  http://localhost:3301;
        }
    }
}

[manucorporat]

I would love to merge this PR, but what's the use case? i don't think anyone should use the stencil's dev server in production

[LeviticusMB]

My main motivation was simply that I needed to expose a couple of API endpoints that my app requires and this was the easiest way to do that. (But see #896 and #1252)

The fact that I can make my developer environment use exactly the same parameters as the production environment (caching, compression, http/2 instead of https, tls config, redirects and URL aliases etc.) is just a very welcome bonus.

[adamdbradley]

Thanks for the PR, merged