rpc: add rpc v2 chainSpec to polkadot (#2859)

The [chainSpec RPC API from the v2
spec](https://paritytech.github.io/json-rpc-interface-spec/api/chainSpec.html)
was only added to substrate-node and should be added to polkadot as well

/cc @lexnv

polkadot/node/core/candidate-validation/src/tests.rs

@@ -726,7 +726,7 @@ fn candidate_validation_retry_on_error_helper(
 		ExecutorParams::default(),
 		exec_kind,
 		&Default::default(),
-	));
+	))
 }
 
 #[test]