feat: update enterprise customer serializer to include additional fields

[katrinan029]

Description

Update EnterpriseCustomerSerializer in the edx-enterprise repo to return these additional fields: country, default_language, and enable_slug_login,

JIRA: https://2u-internal.atlassian.net/browse/ENT-9169

Test

1. checkout this branch
2. visit this link: http://localhost:18000/enterprise/api/v1/enterprise-customer/
3. verify that the new fields are in the enterprise customer object

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.
• [x] Version bumped
• [x] Changelog record added
• [ ] Translations updated (see docs/internationalization.rst but also this isn't blocking for merge atm)

Post merge:

• [x] 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.
• [x] 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.