Add call to generate edX usernames on course run enroll if none exist

[jkachel]

What are the relevant tickets?

Closes mitodl/hq#10536

Description (What does it do?)

Saving an enrollment through the regular v1/enrollments API more-or-less just calls create_run_enrollments, which bails out if there's no edX username. This PR adds a check to see if there is a username for the user, and creates only the username if there isn't one.

How can this be tested?

Automated tests should pass.

Test by:

1. Sign into Learn with a new user. Do not sign into MITx Online.
2. Ensure there's no OpenEdxUser for the new user. (The user shouldn't exist at all in MITx Online.)
3. From Learn, attempt to enroll in a standalone course for audit. This should hit the /api/v1/enrollments endpoint, and that should work.
4. Check to see that an OpenEdxUser is now created for the user. (It may or may not be synced depending on whether or not you have edX/Tutor set up and running.)
5. Check to see that there's an enrollment for the user. There should be an audit enrollment for the course run. (Again, it may or may not be synced yet.)

Additional Context

This doesn't do anything for program enrollments because those don't hit edX.

It only makes the edX username because the create_run_enrollments call will ultimately try to repair the user - it just won't get to that point unless there's a username set for them. So, since the repair call always happens, there's already a call to create the user, so there's no need to do it prematurely.

[github-actions]

OpenAPI Changes

Show/hide ## Changes for v0.yaml:

## Changes for v0.yaml:


## Changes for v1.yaml:


## Changes for v2.yaml:

Unexpected changes? Ensure your branch is up-to-date with main (consider rebasing).