Update Item, CancellationTerm, ReturnTerm, and ReplacementTerm schema…

[charu1605]

The changes involve:

1. Modifying the structure of the cancellation_terms, return_terms, and replacement_terms to better reflect the relationship between different policies.
2. Adding a fulfillment_managed_by field to the ReturnTerm and ReplacementTerm schemas to define who manages the return/replacement process (Consumer or Provider).
3. Clarifying descriptions for various attributes, improving consistency across the schemas.
   These changes aim to align the schemas with the intended functionality and improve their clarity for better understanding and future implementations.