Provide better control for when to render HAL items as Collections

Now there is only the flag to enforce embedded collections for the HalResourcesSerializer but that is limiting and only apply to Resources.

Though I would run the idea by here on converting the simple flag to a plugin solution that would be used by HalResourcesSerializer and HalLinkListSerializer.

One option would be to extend the RelProvider interface with the method:
boolean enforceCollectionsResourceRelFor(Class<?> type);
And because the collection rendering would be a bit to HAL specific for the generic RelProvider interface another interface similar to CurieProvider would define this method:
boolean enforceCollectionForRel(String rel);

That would make it possible for the OptionalListJackson2Serializer and the HalEmbeddedBuilder to consult those methods per type and rel. Does this sound reasonable?

Start by reading HalResourcesSerializer, HalLinkListSerializer, OptionalListJackson2Serializer, and HalEmbeddedBuilder, then inspect the RelProvider and CurieProvider interfaces. Determine how collection rendering is currently controlled and define the plugin or interface behavior for resource types and rels; done means both serializers honor the chosen per-type or per-rel policy.