Support composite or calculated fields

[hippiehunter]

This needs to be implemented with #35 in mind. One of the possible answers is to use an attribute defined in the user code that indicates the composite or calculated field has a dependency on some other set of underlying data.

Aha! Link: https://synergex.aha.io/features/HC-90

[SteveIves]

Note that all properties MUST have a setter in order to be supported, so the setter for these properties should throw. But if that is all we do then Web API will turn it into a 500 which is not appropriate. We need to figure out how to make this throw a 400 bad request. Maybe something in the Web API pipeline to do this?