feat: add PERSISTENT_SUBSECTION_GRADE_CHANGED

[ormsbee]

Implementation of #542

Description

We already have a PERSISTENT_GRADE_CHANGED event, but we do not have a corresponding event for the subsection.

This mostly borrows the field data already calculated for openedx-platform's SUBSECTION_GRADE_CALCULATED tracking event.

I don't love the fact that we are putting the model name in the event name (PersistentSubsectionGrade), but I think it's better to be consistent with the already existing convention for emitting the course grade change event.

Checklists

Check off if complete or not applicable:

Merge Checklist:

• All reviewers approved
• Reviewer tested the code following the testing instructions
• CI build is green
• Version bumped
• Changelog record added with short description of the change and current date
• Documentation updated (not only docstrings)
• Integration with other services reviewed
• Fixup commits are squashed away
• Unit tests added/updated
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

Post Merge:

• Create a tag
• Create a release on GitHub
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)
• Upgrade the package in the Open edX platform requirements (if applicable)

[ormsbee]

@bmtcril: May I trouble you to take a pass at this? I'm not sure what the Avro evolvable schema errors are about, but I'll look into it tomorrow. I just wanted to get a sanity check on the payload and comments. I still need to hook this up to an openedx-platform branch and test to make sure we're really spitting out what I think we're spitting out, especially for the visible_blocks field and weight.