Subscription based commands and lazy attributes

[mapedraza]

Describe the solution you'd like

Right now, commands (and lazy attributes) relies on NGSI registrations. Since NGSI registration support is not the best (telefonicaid/fiware-orion#4337), and subscriptions are more powerful, the idea of this issue is to make the IoTAs compatible to subscription based commands and lazzy attributes.

There is a twin issue in Orion: telefonicaid/fiware-orion#4397

Describe alternatives you've considered

Registrations (existing one)