Support keyless entities for singletons

Entity types that are used for singletons, or targets of a single-valued containment navigation property, don't need keys. Keys are only required when accessing an entity within a collection.

We could consider relaxing the requirement that entity types used exclusively for singletons/single-valued containment nav props have keys. We would have to carefully think through the ramifications of doing this, but they should always be able to be accessed through their canonical URL without a key.