Allow derived class content to be serialized/deserialized correctly in all publishers

[jeremyiverson]

• XML: Choice group, with name = $"{ConcreteTypeName}{PropertyName}"
• JSON: Output type property with the concrete type name

[DanSmith]

for xml publisher, also investigate using xsi:type declarations

[DanSmith]

As of e677cf3 the json schema, csharp, and csharp json serializer support this. Xml and csharp json deserialization are still to be done.

The on disk format of the csv must be updated to the current version with new columns by running the cogs rewrite command, and the AllowSubtypes column for a property should be set to true where type discriminators are to be allowed.

[DanSmith]

As of 4f736e2 the csharp target will parse type discriminated json.

Review of XML is still needed