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Problem/Feature

As explained in the official documentation for the GET Drops Entitlements API, fulfillment_status is one of the possible query parameter that the API is accepting.
Until now, this parameter was ignored in the mock-api. This PR aims to fix this, so now the mock-api supports filtering by fulfillment_status.

Description of Changes:

• Read the "fulfillment_status" optional query parameter from the URL
• Use this new field as a value to the DropsEntitlement object used for the DB query

Checklist

• My code follows the Contribution Guide
• I have self-reviewed the changes being requested
• I have made comments on pieces of code that may be difficult to understand for other editors
• I have updated the documentation (if applicable)