Implement horizontal path-based HTTP routes
Horizontal here means "across all websites", think for instance of .well-known URLs that we want to serve on all hosted domains.
The implementation will likely take advantage of the site-common.conf NGINX config snippet that is already included at top-level in all server directives. If we parameterize the force-to-https behavior (and install non-forced-https handlers in the nginx default vhost), we could move the ACME handlers on this mechanism.
On the service configuration side, this feature will require either another class of public_endpoint, or a specialization of the public_http_endpoint with a custom attribute (or perhaps a convention like "name=*").