feat(Managers): add customizable caching for managers (#6013)

src/managers/CachedManager.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const DataManager = require('./DataManager');
+
+/**
+ * Manages the API methods of a data model with a mutable cache of instances.
+ * @extends {DataManager}
+ * @abstract
+ */
+class CachedManager extends DataManager {
+  constructor(client, holds, iterable) {
+    super(client, holds);
+
+    Object.defineProperty(this, '_cache', { value: this.client.options.makeCache(this.constructor, this.holds) });
+
+    if (iterable) {
+      for (const item of iterable) {
+        this.add(item);
+      }
+    }
+  }
+
+  /**
+   * The cache of items for this manager.
+   * @type {Collection}
+   * @abstract
+   */
+  get cache() {
+    return this._cache;
+  }
+
+  add(data, cache = true, { id, extras = [] } = {}) {
+    const existing = this.cache.get(id ?? data.id);
+    if (cache) existing?._patch(data);
+    if (existing) return existing;
+
+    const entry = this.holds ? new this.holds(this.client, data, ...extras) : data;
+    if (cache) this.cache.set(id ?? entry.id, entry);
+    return entry;
+  }
+}
+
+module.exports = CachedManager;