[SWIFT4] Objective C compatibility

Created by: Sal0g

Currently the generator (v3.3.3) produces Swift 4.2 code that is not directly useable in Mixed Swift/Objective C projects even though the config files contains:

"objcCompatible":"true",

• Currently the generator produces Struct objects which are not accessible in Objective C. It would instead produces @objc public class CLASSNAME: NSObject, Codable { ... }
• everywhere where public is produced, it would be necessary to have @objc public as in Swift 4, such automatic @objc inference is deprecated and disabled by default