Implement integer source units and chunk payloads

internal/framework/pipeline/testdata/walking_skeleton_input.json

@@ -2,15 +2,15 @@
   "id": "fixture-source",
   "units": [
     {
-      "id": "u1",
+      "id": 1,
       "text": "First event."
     },
     {
-      "id": "u2",
+      "id": 2,
       "text": "Second event."
     },
     {
-      "id": "u3",
+      "id": 3,
       "text": "Third event."
     }
   ]