Initial commit: Pezkuwi SubQuery indexer

- pezkuwi.yaml: Relay chain staking indexer (rewards, slashes, pools, transfers, era info)
- pezkuwi-assethub.yaml: Asset Hub indexer (NominationPools, asset transfers)
- GraphQL schema for staking data entities
- Handler mappings from Nova SubQuery base

scripts/python_scripts/singleton.py

@@ -0,0 +1,6 @@
+class Singleton(type):
+    _instances = {}
+    def __call__(cls, *args, **kwargs):
+        if cls not in cls._instances:
+            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
+        return cls._instances[cls]