fs: implement --metadata-mapper to transform metatadata with a user supplied program

bin/test_metadata_mapper.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python3
+"""
+A demo metadata mapper
+"""
+
+import sys
+import json
+
+def main():
+    i = json.load(sys.stdin)
+    # Add tag to description
+    metadata = i["Metadata"]
+    if "description" in metadata:
+        metadata["description"] += " [migrated from domain1]"
+    else:
+        metadata["description"] = "[migrated from domain1]"
+    # Modify owner
+    if "owner" in metadata:
+        metadata["owner"] = metadata["owner"].replace("domain1.com", "domain2.com")
+    o = { "Metadata": metadata }
+    json.dump(o, sys.stdout, indent="\t")
+
+if __name__ == "__main__":
+    main()