fix: Add CountryFieldMixin to the EnterpriseCustomer serializer

[brobro10000]

In stage, the enterprise dashboard list call found on the enterprise admin portal is not populating with enterprise customers. Looking into the logs we are return the following error.

We add the CountryFieldMixin to the EnterpriseCustomer serializer to resolve the issue. https://github.com/SmileyChris/django-countries?tab=readme-ov-file#django-rest-framework

Merge checklist:

• [ ] Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • base.in if needed in production but edx-platform doesn't install it
  • test-master.in if edx-platform pins it, with a matching version
  • make upgrade && make requirements have been run to regenerate requirements
• [ ] make static has been run to update webpack bundling if any static content was updated
• [ ] ./manage.py makemigrations has been run
  • Checkout the Database Migration Confluence page for helpful tips on creating migrations.
  • 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.
  • This should be run from either a venv with all the lms/edx-enterprise requirements installed or if you checked out edx-enterprise into the src directory used by lms, you can run this command through an lms shell.
    • It would be ./manage.py lms makemigrations in the shell.
• [ ] Version bumped
• [ ] Changelog record added
• [ ] Translations updated (see docs/internationalization.rst but also this isn't blocking for merge atm)

Post merge:

• [ ] Tag pushed and a new version released
  • Note: Assets will be added automatically. You just need to provide a tag (should match your version number) and title and description.
• [ ] After versioned build finishes in GitHub Actions, verify version has been pushed to PyPI
  • Each step in the release build has a condition flag that checks if the rest of the steps are done and if so will deploy to PyPi. (so basically once your build finishes, after maybe a minute you should see the new version in PyPi automatically (on refresh))
• [ ] PR created in edx-platform to upgrade dependencies (including edx-enterprise)
  • Trigger the 'Upgrade one Python dependency' action against master in edx-platform with new version number to generate version bump PR
  • This must be done after the version is visible in PyPi as make upgrade in edx-platform will look for the latest version in PyPi.
  • Note: the edx-enterprise constraint in edx-platform must also be bumped to the latest version in PyPi.

[brobro10000]

Closing out this PR. The missing stage data has been remediated, so no further action in necessary from a code perspective.