[D8] Add support for opt-in CORS

This is part of #378, since this is something possible in D8.2.x: https://www.drupal.org/node/2715637

As of Drupal 8.2, it's possible to opt in a particular site to enable CORS for responses served by Drupal.

(This is particularly helpful for fully decoupled Drupal sites which have JS that needs to talk to a Drupal 8 site's REST API. In such cases, that Drupal 8 instance often runs on a separate domain. Due to the same origin policy those requests will be blocked by the browser.)

This is not enabled by default because there are security consequences.

New in default.services.yml:

  # Configure Cross-Site HTTP requests (CORS).
  # Read https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
  # for more information about the topic in general.
  # Note: By default the configuration is disabled.
 cors.config:
   enabled: false
   # Specify allowed headers, like 'x-allowed-header'.
   allowedHeaders: []
   # Specify allowed request methods, specify ['*'] to allow all possible ones.
   allowedMethods: []
   # Configure requests allowed from specific origins.
   allowedOrigins: ['*']
   # Sets the Access-Control-Expose-Headers header.
   exposedHeaders: false
   # Sets the Access-Control-Max-Age header.
   maxAge: false
   # Sets the Access-Control-Allow-Credentials header.
   supportsCredentials: false

Note particular that enabled: false key-value pair!