Issue #3172194: Images are queued and purged via plugins

This is an improved version of MR !8 created by @jjcarrion.

This merge request provides the following changes:

1. The image purging logic is moved from varnish_image_purge_entity_update() into respective purge module plugins.
2. The change provides the following plugins:
   • uri invalidation plugin
   • varnish_image purger plugin
   • varnish_image queuer plugin
3. When an entity with image fields is updated, the varnish_image_purge.varnish_image_queuer service identifies the image fields, gathers URIs of said image fields, creates the invalidations of type uri and adds to the queue.
4. A varnish_image purger plugin can be configured to process invalidations of type uri. This is the plugin where logic is moved from varnish_image_purge_entity_update() function.
5. Varnish Image Purge configuration form is moved from a separate page to the varnish_image queue plugin configuration form.
6. File varnish_focal_point_purge.module is removed as all image styles are handled by varnish_image_purge module.