Add validation for target_types in google_bigquery_dataset_access (#15446)

Author: wj-chen

mmv1/products/bigquery/DatasetAccess.yaml

@@ -269,6 +269,8 @@ properties:
         required: true
         item_type:
           type: String
+        item_validation:
+          regex: '^[A-Z_]+$'
   - name: 'routine'
     type: NestedObject
     description: |