Add: AI/GS GetMonthlyMaintenanceCost (#6897)

API addition which allows AI/GS scripts to retrieve the monthly maintenance cost of an airport type.

src/script/api/script_airport.cpp

@@ -163,3 +163,10 @@
 
 	return AirportSpec::Get(type)->maintenance_cost;
 }
+
+/* static */ Money ScriptAirport::GetMonthlyMaintenanceCost(AirportType type)
+{
+	if (!IsAirportInformationAvailable(type)) return -1;
+
+	return (int64)GetMaintenanceCostFactor(type) * _price[PR_INFRASTRUCTURE_AIRPORT] >> 3;
+}