assets domain for Easy IO + S3

Folks using S3, GCS, Spaces, and such can only deliver their images with Easy IO. But since we can optimize/deliver CSS/JS and more, it would be useful to give the ability to deliver those from Easy IO via the origin somehow. We may not need a separate domain, just use edge rules with a similar trigger to https://deliciousbrains.com/wp-offload-media/doc/same-cloudfront-domain/