feat: support percentage-based Waffle feature flags by enterprise customer

[adamstankiewicz]

Description

In support of ENT-10429, we'd like to be able to de-risk the experimentation/verification of the proposed approach for concurrent network requests within the BFF API layer in enterprise-access by supporting an incremental, percentage-based rollout.

However, in the current approach to enterprise feature flags, the percentages set in our Waffle flags are applicable globally to all users. They do not allow enabling a percentage-based flag for a specific enterprise customer(s).

This PR adds support to create percentage-paged feature flags, exposed as a sibling to the existing enterprise_features returned in the /enterprise/api/v1/enterprise-learner/ API.

Example:

{
  "enterprise_features": {
    // ...existing features, e.g. `"catalog_query_search_filters_enabled": true`
  },
  "enterprise_features_by_customer": {
      "01b54903-bdf1-4830-85ed-beb421b37ff4": {
          "enterprise_learner_bff_concurrent_requests": true
      }
  }
}

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.