Don't respond to .sxg/* when request host is html_host

[twifkak]

To minimize conflict with the origin's URL namespace, the cloudflare_worker could only respond to reserved_path URLs when requested on the workers.dev domain. Does fastly_compute have a similar special-purpose domain? If not, this could be an optional behavior.

[antiphoton]

Fastly has such domain, which is edgecompute.app.