Allow id in fragment

Hello,

I would request to allow a (optional) id field for the fragment.
The reason for this request is, that it seams to be the most suitable clean solution to handle compositions with ember/JSONAPI.
I started a discussion without much participation about this problem here: https://discuss.emberjs.com/t/ember-data-handling-composition/15035/2

When allowing fragments to have an id attribute, they would solve the problem of compositions without changes at JSONAPI and backend frameworks in a nice way.