feat(iar): update Django, add MultiSelectCustomField field, refactor models, add migration
- Update Django version in requirements/base.txt from 3.2.12 to 4.2.0.
- Introduce a new custom field, MultiSelectCustomField, in iar/custom_fields.py, extending MultiSelectField with a custom validator adjustment method.
- Replace all instances of MultiSelectField with MultiSelectCustomField and models.NullBooleanField with models.BooleanField in iar/models.py.
- Add new migration 0002_alter_asset_data_category_alter_asset_data_subject_and_more.py to reflect the updates in models and custom field usage.