fix mlm:artifact_type check missing + update corresponding tests/examples

[fmigneault]

Description

Alternate to #52 using $schema with draft07.
Requires a manual copy of the fields rather than taking advantage of reusing allOf schemas categorizing fields by shared/item-only/asset-only.

Related Issue

• fixes Fix missing mlm:artifact_type in JSON schema #42

Type of Change

• 📚 Examples, docs, tutorials or dependencies update;
• 🔧 Bug fix (non-breaking change which fixes an issue);
• 🥂 Improvement (non-breaking change which improves an existing feature);
• 🚀 New feature (non-breaking change which adds functionality);
• 💥 Breaking change (fix or feature that would cause existing functionality to change);
• 🔐 Security fix.

Checklist

• I've read the CONTRIBUTING.md guide;
• I've updated the code style using make check;
• I've written tests for all new methods and classes that I created;
• I've written the docstring in Google format for all the methods and classes that I used.