Add write-only serializer for EntityMetadata

[jviau]

Adds a custom write-only System.Text.Json JsonConverter for EntityMetadata<TState> to allow for serializing this type in responses. This is write-only at the time as we do not see value in reading this type, especially since the EntityMetadata derived type needs the DataConverter supplied to it. Supporting write has value as without a custom converter, accessing .State could throw depending on the value of IncludesState.