GET Drops Entitlements "mock-API" - Enabled filter by status

[frifugio]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

As explained in the official documentation for the GET Drops Entitlements API, _fulfillmentstatus 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 _fulfillmentstatus.

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

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