add polymorph support to deserialization

[jakobharder]

There are two examples in the test cases:

[Polymorph(typeof(ValueItem), "value/id")]
[Polymorph(typeof(ComplexItem), "None/ObjectId")]
public BaseItem[]? List { get; set; }

[Polymorph(typeof(ElementType0), "ElementType", HexValue = "00000000", Node = PolymorphRootNode.Parent)]
[Polymorph(typeof(ElementType1), "ElementType", HexValue = "00000001", Node = PolymorphRootNode.Parent)]
public Element? Element { get; set; }

[taubenangriff]

lgtm