Add check for cache backend.

Problem/Motivation

The site may still rely solely on the default database or file-based cache backends, instead of using a dedicated in-memory cache backend such as Redis or Memcached, which can limit performance and scalability under higher load.

Steps to reproduce

Proposed resolution

Add a new Performance FitCheck plugin CachedBackendCheck that:

• Inspects the configured cache backends for key bins (for example, render, dynamic_page_cache, discovery) to determine whether an in-memory backend like Redis or Memcached is in use.
• Fails with FitWeight::Medium when a production environment is not using a recommended cache backend, and passes with FitWeight::Ok when Redis/Memcached (or equivalent) is configured.
• Provides a short help message suggesting configuring Redis or Memcached as the primary cache backend for improved performance and scalability.

Remaining tasks

User interface changes

API changes

Data model changes