Reintroduce instigating paths for snapshot middleware

src/snapshot_middleware/snapshots/test__csv_from_imfs.snap

@@ -5,6 +5,8 @@ expression: instance_snapshot
 snapshot_id: ~
 metadata:
   ignore_unknown_instances: false
+  instigating_source:
+    Path: /foo.csv
   relevant_paths:
     - /foo.csv
 name: foo
@@ -13,4 +15,4 @@ properties:
   Contents:
     Type: String
     Value: "[{\"key\":\"Ack\",\"example\":\"An exclamation of despair\",\"source\":\"Ack!\",\"values\":{\"es\":\"¡Ay!\"}}]"
-children: []
+children: []