Structured - core - change StructuredData to StructuredDataDictionary

[sgryphon]

Implement IDictionary<string, object> interface, and expose through that.
MessageTemplate is exposed as property "MessageTemplate" in the dictionary (similar to "Exception").
Want to keep as separate class for the string rendering.
Also support "MessageTemplate" property when rendering.

Maybe change IStructuredData to only have Item[string] property, or to simply be a subtype of the dictionary (i.e. enforce other implementer to follow property naming) - but still want explicit so that listeners can distinguish and exclude other dictionaries if they want (but still want interface, so not bound to concrete class).