Improved Ember Data and Ember Changeset support for relationships

### Main problem
Create a changeset with a `model`, set a belongsTo relationship, change a belongsTo, and lastly revert back to original - the changeset will be dirty

Support for ember-data relationship objects has been fraught with issues. Precisely and often with async (default) relationships in Ember Data. This is due to them being behind a Proxy, thus a `===` will fail with the same relationship.

This issue is to track (and keep me honest) in either implementing better support for ember data relationships or improved documentation.

One idea that is floating around is to use @embroider/macros to see if e-d is installed and then we can `importSync` the model and do an `instanceof Model` check. I'll be exploring this space in the next few weeks.