Allow extensions to access the configured directus cache

Summary Extensions may benefit from using a cache for various reasons (caching, synchronization across instances (e.g. locking mechanisms)). However, accessing the default directus cache is not straightforward and requires reimplementing the [getCache]( function. Ideally, a preconfigured cache could be provided to extensions. Basic Example Caching Simple endpoint that uses the cache to cache responses of an upstream API, but limits access to users who are logged in. Locking Hook extensions that triggers a long-running action. Motivation Allowing caching in extensions using the interface that directus already provides. This makes the implementation for extension developers much easier, safer and incurs less overhead because we use existing libraries instead of bundling our own. Detailed Design Directus provides detailed configuration options for the cache. See The extensions cache could be configured the same way: | Variable | Description | Def…