feat: Changes to Item attributes in draft spec

[rashap224]

Issue Reference : Resolves #321
In draft spec here, request the following changes:

• Move return_eligible to Item.return_terms
• Move replacement_eligible to Item.replacement_terms
• Add fulfillment_managed_by to Item.replacement_terms

Reason:
Return & replacement of items is independent of cancellation. Cancellation of a fulfillment can result in return-to-origin (RTO) if the buyer refuses to accept it, irrespective of whatreturn_eligibleis. On the other hand, fashion items are generally returnable while some items (that are made-to-order e.g., cake) can't be returned, hence return_eligible will be false.