Refactor `StackItem` to use Jackson annotations for deserializing subclasses

The StackItem class currently uses a custom deserializer to deserialize its various subclasses. See TransactionAttribute as an example on how to do this.

[mialbu]

I tried to do this and it works for all StackItemTypes except for Maps. For the StackItemType Map, a custom Key deserialiser is needed.

Reason for this: In a Map-StackItem the value is of type Map<StackItem, StackItem>, hence the key-value pair are both StackItems. Using Jackson annotations, however, by default the keys of a Map are deserialised as Strings. This leads to an InvalidDefinitionException that asks for a (Map) Key deserialiser.

[mialbu]

This was merged. I’ll close it.