D8 Clear external caches (varnish, etc.)

We need to port #2281597: Clear external caches (varnish, etc.)
to 8.x

another similar 7.x module here: https://www.drupal.org/project/clear_varnish_page_cache

had a look at how to do this.

Depends on which version of varnish in use.

My client currently uses purge and varnish_purger (aka varnish_purge)

would be nice to support both.