Allow rewrites to send response headers

[timkelty]

For example, this may be used to control the Access-Control-Allow-Origin header for CDN assets:

rewrites:
  - pattern:
      hostname: 'cdn.mydomain.com'
      pathname: '/assets/:assetPath+'
    destination: '{assetBaseUrl}/{matches.pathname.groups.assetPath}{request.url.search}'
    headers:
      Access-Control-Allow-Origin: 'mydomain.com'

[timkelty]

This is now implemented and documented here: https://craftcms.com/docs/cloud/redirects.html#custom-headers