Add an annotation to indicate 'always use id'

[cowtowncoder]

Current behavior for Object Id serialization is to serialize first instance normally, and then serialize all the other references as ids. This makes sense when data must be self-contained, i.e. fully deserializable without any external information.

However, there are cases where it would make sense to serialize ALL instances as just id; this makes sense if deserialization side knows how to map these ids externally to instances, without requiring them to be included in serialization.

[cowtowncoder]

Implemented by adding a new annotation:

'@JsonIdentityReference`

to be used on property. Created separate annotation so that basic identity info definition can be separate from details of how references are configured.