Added CORS support for rev-proxy APIs

[siexp]

Added new flag allow_cors: <bool> which enables Cross-Origin requests for proxied API. Without this flag enabled only requests from the same to Api host domains are allowed in browser.

nginx_revproxy_sites:
  example.com:
    ...
    allow_cors: <bool | default( false )>

When enabled force nginx to add following headers to response

   add_header 'Access-Control-Allow-Origin' $http_origin;
   add_header 'Access-Control-Max-Age' 600;

Access-Control-Allow-Origin set to $http_origin which is equal to value from Origin header in request
Access-Control-Max-Age set to 10 min period accordingly to Chromium v76 and below cap

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Max-Age