Support the 'add_existing_widget' setting from #2683125

Here's a patch, although it should probably be postponed on ✨ Allow select widget for "Add existing items" Needs work landing. 😅

Hiding patch, there's an MR for this now. Although there's no CI configured for this project, either DrupalCI nor GitLab CI, so it kinda doesn't matter. 😉

Thanks! Would it make sense to bring this feature into IEF - Complex Open until/if it gets accepted into IEF?

Brought in (as indented by ✨ Hide the 'Add existing [label]' form / button if there are none Needs review ).

Thanks!