Add a hook for retrieving ActiveRecord fragments for included relationships

For example, if you have a resource UserResource with a method like this

def friends
   @model.friends.size
end

It will load user.friends separately for each model in the list. So you can optimise the query like this:

class self.records
    _model_class.all.includes(:friends)
end

Then when requesting GET /users/ it now works well. However, say if you request something like GET /groups/?include=users, the records method on UserResource is never called, so there is no query optimisation.

Instead of overriding self.apply_includes(records, options = {}) on each resource that has a relationship to UserResource, it would be nice to have a way to do this only once per resource.

@lgebhardt as discussed on Gitter

Start by tracing how GET /groups/?include=users reaches UserResource, and compare the records and apply_includes paths described in the issue. The work is done when one resource-level hook can optimize included relationships without overriding apply_includes on every related resource, while avoiding separate queries for each model.