add postgres persistence support

Author: mani5112

pydantic_graph/pydantic_graph/persistence/postgres.py

@@ -281,4 +281,5 @@ async def load_all(self) -> list[Snapshot[StateT, RunEndT]]:
         raw_payloads = [r['snapshot'] for r in rows]
         payloads = [json.loads(p) if isinstance(p, str) else p for p in raw_payloads]
         assert self._snapshot_adapter is not None, 'Persistence types not set'
-        return self._snapshot_adapter.validate_python(payloads)
+        return self._snapshot_adapter.validate_python(payloads)
+