add missing item_assets check + remove mlm:name unique requirement

[fmigneault]

Description

Added

• Add the missing JSON schema item_assets definition under a Collection to ensure compatibility with
  the Item Assets extension, as mentioned this specification.

Removed

• Field mlm:name requirement to be unique. There is no way to guarantee this from a single Item's definition
  and their JSON schema validation. For uniqueness requirement, users should instead rely on the id property
  of the Item, which is ensured to be unique under the corresponding Collection, since it would not be retrievable
  otherwise (i.e.: collections/{collectionID}/items/{itemID}).

Related Issue

n/a

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 codestyle;
• 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.