[Persistence] Begin adding example bundle

Begin adding example bundle illustrating usage of secondary persistence
stores. Scratchpad will 'persist' objects only in memory.

example/scratchpad/README.md

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+Example of using multiple persistence stores by exposing a root
+object with a different space prefix.

example/scratchpad/bundle.json

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+{
+    "extensions": {
+        "roots": [
+            {
+                "id": "scratch:root",
+                "model": {
+                    "type": "folder",
+                    "composition": [],
+                    "name": "Scratchpad"
+                },
+                "priority": "preferred"
+            }
+        ]
+    }
+}