This commit introduces a new cache driver for APCu (APC User Cache).

Reasoning:
APCu is the modern, actively maintained successor to the old APC extension. It provides a significant performance improvement over the default file-based caching mechanism, which is especially beneficial for busy stores. Adding this driver gives store owners with modern server environments a high-performance caching option out-of-the-box.

Implementation Details:

    The driver implements all required methods of the cache interface: get, set, delete, and deleteAll.

    It properly checks if the apcu PHP extension is loaded before attempting to use it.

    It respects the CACHE_PREFIX constant to avoid potential key collisions on shared servers.

    The deleteAll method (used for clearing cache by prefix) is implemented efficiently using APCIterator.

…nality verification

- Replaced APCUIterator constructor call with correct signature (pattern + APC_ITER_KEY)
- Performed complete end-to-end validation of all cache operations (get/set/delete/clear/list)
- Verified stable behavior on PHP 7.4–8.4 with APCu enabled

This version represents a final, production-ready implementation of the APCu cache driver based solely on APCUIterator, without any external registry.