[FR] Support polymorphic relationships

[egmacke]

Is your feature request related to a problem? ^*

Sometime a relationship may be polymorpic. For example, the relationship of vehicle could be a Car or a Lorry.

Currently there is no way to provide different relationship serializers based on the value of the relationship.

Describe the solution you'd like^*

When defining the relator serializer, it would be good to be able to pass a collection of serializers with a discriminator field.