EventIndex: Add a method to get the current disk usage of the index.

src/indexing/BaseEventIndexManager.js

@@ -117,6 +117,13 @@ export default class BaseEventIndexManager {
         throw new Error("Unimplemented");
     }
 
+    /**
+     * Get the disk usage of the index
+     */
+    async indexSize(): Promise<number> {
+        throw new Error("Unimplemented");
+    }
+
     /**
      * Commit the previously queued up events to the index.
      *