Enhancement: Add after_wpsc_delete_cache_directory hook

### Impacted plugin

Super Cache

### What

Add after_wpsc_delete_cache_directory hook

### How

Added after_wpsc_delete_cache_directory hook, which is executed every time someone hits button to clear cache. This is inteded to integrate SuperCache with other Caching providers, eg. Cloudflare.

Main goal to add this, was to enable purge entire CF Cache (or only one page) even, when no WP Super Cache is present (in that case, wp_cache_cleared hook isn't executed.

Code is in https://github.com/Automattic/jetpack/pull/25865

Review the proposed implementation in Automattic/jetpack PR 25865 and trace the Super Cache cache-clear button flow. Confirm the hook is invoked whenever the cache directory is deleted and that it supports integrations such as Cloudflare, then verify the relevant existing behavior.

Written by the indexing model from the issue text.