RPC: expose chain spec properties (#1104)

* Add properties to chain spec

* Read properties as serde_json::Value

* Use a serde json::map::Map directly for properties

* Add type alias for json Map

* Update chain_spec.rs

substrate/core/rpc/src/system/tests.rs

@@ -27,6 +27,9 @@ impl SystemApi for () {
 	fn system_chain(&self) -> Result<String> {
 		Ok("testchain".into())
 	}
+	fn system_properties(&self) -> Result<serde_json::map::Map<String, serde_json::Value>> {
+		Ok(serde_json::map::Map::new())
+	}
 }
 
 #[test]
@@ -52,3 +55,11 @@ fn system_chain_works() {
 		"testchain".to_owned()
 	);
 }
+
+#[test]
+fn system_properties_works() {
+	assert_eq!(
+		SystemApi::system_properties(&()).unwrap(),
+		serde_json::map::Map::new()
+	);
+}