feat: add created time to subscriptions api response

[katrinan029]

Description

include created time in subscriptions api response api/v1/subscriptions/

Link to the associated ticket: https://2u-internal.atlassian.net/browse/ENT-9374

Testing considerations

1. Follow the set up steps here https://github.com/openedx/license-manager to get the server running
2. checkout this branch with git fetch -a and git checkout knguyen2/ent-9374
3. visit http://localhost:18170/login and get the customer uuid from any subscription plan http://localhost:18170/admin/subscriptions/subscriptionplan/. create a plan if one doesn't exist
4. copy the customer uuid and paste in the following link: https://license-manager-internal.stage.edx.org/api/v1/subscriptions/?enterprise_customer_uuid=<CUSTOMER_UUID>
5. verify that you see the created date
   

• Include instructions for any required manual tests, and any manual testing that has
  already been performed.
• Include unit and a11y tests as appropriate
• Consider performance issues.
• Check that Database migrations are backwards-compatible

Post-review

Squash commits into discrete sets of changes

[pmakwana93]

lgtm