Advanced events and subscriptions management

[erika-rosaverde]

• I created two interfaces (event_handler_interface.EventHandlerInterface and subscription_handler_interface.SubscriptionHandlerInterface) to handle more implementations of the events and subscriptions handlers.
• I implemented the redfish_event_handler that manages NewAggregationSourceDiscovered and ResourceCreated.
• I created object_handler.ObjectHandler to execute custom logic to handle specific types of objects requests.