feat: make customer agreements fields available in learner license en…

[jajjibhai008]

Description

Make the newly added fields in the customer agreement model available in learner license endpoint.

endpoint: /api/v1/learner-licenses?enterprise_customer_uuid=the-uuid&active_plans_only=true&current_plans_only=false

Description

Description of changes made

Link to the associated ticket: https://openedx.atlassian.net/browse/ENT-XXXX

Testing considerations

• 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