Support EventNotifications with singleton related objects

[xavdid]

Why?

There are a few event notification types whose related object is a singleton. As a result, its id field will always be null. Rather than mark the root RelatedObject class as having a nullable string for its id (causing every user to have to do null checks even when we know it'll be there), we're adding a second class and generating accordingly. There'll never be any ambiguity as ot whether a id field will be present in the response.

What?

• add RelatedSingletonObject class, which is a copy of RelatedObject, but without an id. I didn't do inheritance (when relevant) because it's such a tiny, colocated class. If the repetition bothers us, we can reassess.
• add related methods, as needed
• tests

See Also

• RUN_DEVSDK-2825