Add duo_default_namespace in model

What does this MR do and why?

This is a continuation of !204649 (merged).

1. override reader method
2. override writer method so it would conditionally update default_duo_add_on_assignment_id instead
3. custom validation

There will be further MRs based on this to implement API and web UI.

References

#557584

How to set up and validate locally

Currently the spec serves as coverage on various calls to this new attribute.

MR acceptance checklist

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Related to #557584