feat: include created time in policy api response

[katrinan029]

Description

include created time in policy api response api/v1/subsidy-access-policies/?enterprise_customer_uuid=<uuid>

Testing instructions

1. Follow the set up steps here https://github.com/openedx/enterprise-access to get the server running
2. checkout this branch with git fetch -a and git checkout knguyen2/ent-9374
3. visit http://localhost:18270/admin/ and click on any learner credit to get the customer uuid i.e. http://localhost:18270/admin/subsidy_access_policy/assignedlearnercreditaccesspolicy/22424a81-ddda-4af0-a122-aa32b5d624c9/change/
4. copy the customer uuid and paste in the following link: http://localhost:18270/api/v1/subsidy-access-policies/?enterprise_customer_uuid=<CUSTOMER_UUID>
5. verify that you see the created date
   

Jira:
ENT-9374

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance

[pmakwana93]

lgtm