feat: add PERSISTENT_SUBSECTION_GRADE_CHANGED

[bmtcril]

Description

(Rebase of #545 )

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.

This event is necessary for CBE work, which will react to these events in order to determine competency at a skill.

Still needs testing in platform, so in draft.

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)