[dasc] Support add / remove child objects

As an author, I would like the ability to work with my singleton child objects in a way that is consistent with the add / remove array feature.

## Criteria of acceptance
1. Ability to add an object
2. Ability to remove an object
3. Leans on validation framework for errors

## Additional context
I'm seeing a theme here that for now, we will let authors shoot themselves in the foot and we will lean on the validation framework to tell them what should be done. This will get us out of having to build additional UIs around telling an author, "I'm sorry, this is a required field." We let them remove, and we lean on the foundational validation UI to handle when something is not valid. They can then, again, use existing UI affordances to add a required field back.

The issue names no files or tests; start by locating the existing add/remove array feature and the code handling singleton child objects. Confirm how the validation framework reports invalid required fields, then implement matching add and remove behavior with validation errors covering the stated acceptance criteria.

Written by the indexing model from the issue text.