feat: share durable dataset services across route modules

src/services/datasets.ts

@@ -0,0 +1,6 @@
+/** Shared composition root for the dataset HTTP surface. */
+import { DatasetService } from "./dataset-service";
+import { DatasetVersionService } from "./dataset-version-service";
+
+export const datasetService = new DatasetService();
+export const datasetVersionService = new DatasetVersionService({ datasetService });