Add arity key on field data returned by jsonapi

gaëlg → made their first commit to this issue’s fork.

gaëlg → changed the visibility of the branch 3275497-add-arity-key to hidden.

I need to fix related tests.

The MR adds the arity key to every resource even if not necessary (for example on responses containing a single resource).
It keeps the code simple and more predictable, but it means a lot of tests had to be updated.

Left comments/questions in MR.

I answered the questions.
If this needs fixing, we should probably fix it in JsonApiResource as well?

All the tests having to be updated to pass makes me think we are introducing a breaking change

Kinda depends, adding a meta to add responses does mean we are doing a change. Is it breaking? If your client support jsonapi it shouldn't be, its part of the spec.

So i lean towards not breaking.

Can we add an example where it’s not passed to make sure both ways are covered?

It's late. Is this also for creating entities? If so, then it should also work without, otherwise it is indeed a BC break since you might not do that :x