[RFC] Invalidate by Path with ACSF multi-domains support.

I have a project on Acquia Cloud Site Factory, it has at least two domains, a public-facing domain that forbids the access to /user and /admin paths, and a back-office domain that restricts access from allowed IPs.

There is a need from the user to be able to manually purge the cache from Acquia Varnish wich PurgeBlock, but the PurgeBlock only supports purge the URL of the current domain, which is the back-office domain at this setup, but what the user wants is actually to purge the corresponding path from the public-facing domain.