Provide a helper to define custom seeding steps for readability

[lolmaus]

Non-technical staff may have hard time working with "objectIds": [{"id": "1", "type": "product-association"}] format.

To make seeding steps more readable, we might want to provide a helper to define custom seeding steps. The helper would serve the following purposes:

• Map attribute names to more human-readable and feature-focused equivalents.
• Define the type of each attribute/relationship for:
  • Runtime type checking.
  • Avoid the visual noise of quotes, curlies and square brackets, which were previously necessary for guessing types.
• Provide a simple, readable way to specify polymorphic relationships.
• Offer traits — both Mirage-based and inline.