Feature: GS method to control engine availability for a specific company (#7791)

* Feature: GS method to allow company to use an engine before its introduction date

* Feature: GS method to retire an engine early for a specific company

src/command_type.h

@@ -238,6 +238,7 @@ enum Commands {
 	CMD_DECREASE_LOAN,                ///< decrease the loan from the bank
 
 	CMD_WANT_ENGINE_PREVIEW,          ///< confirm the preview of an engine
+	CMD_ENGINE_CTRL,                  ///< control availability of the engine for companies
 
 	CMD_RENAME_VEHICLE,               ///< rename a whole vehicle
 	CMD_RENAME_ENGINE,                ///< rename a engine (in the engine list)