feat: Implement basic general folder support (#2051)

- Related: #1748

tests/ha/registry/entity/mock.ts

@@ -0,0 +1,48 @@
+import {
+  Entity,
+  EntityCache,
+  EntityRegistryManager,
+} from '../../../../src/ha/registry/entity/types';
+import { HomeAssistant } from '../../../../src/ha/types';
+
+export class EntityRegistryManagerMock implements EntityRegistryManager {
+  protected _cache = new EntityCache();
+  protected _fetchedEntityList = false;
+
+  constructor(data?: Entity[]) {
+    data?.forEach((entity) => {
+      this._cache.set(entity.entity_id, entity);
+    });
+  }
+
+  public async getEntity(
+    _hass: HomeAssistant,
+    entityID: string,
+  ): Promise<Entity | null> {
+    return this._cache.get(entityID);
+  }
+
+  public async getMatchingEntities(
+    _hass: HomeAssistant,
+    func: (arg: Entity) => boolean,
+  ): Promise<Entity[]> {
+    return this._cache.getMatches(func);
+  }
+
+  public async getEntities(
+    hass: HomeAssistant,
+    entityIDs: string[],
+  ): Promise<Map<string, Entity>> {
+    const output: Map<string, Entity> = new Map();
+    for (const entityID of entityIDs) {
+      const entityData = await this.getEntity(hass, entityID);
+      if (entityData) {
+        output.set(entityID, entityData);
+      }
+    }
+    return output;
+  }
+
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public async fetchEntityList(_hass: HomeAssistant): Promise<void> {}
+}