[FC-0040] feat: Add `CONTENT_OBJECT_TAGS_CHANGED` signal

[yusuf-musleh]

This signal is emitted when tags on a content object have changed.

Description: This PR adds a new CONTENT_OBJECT_TAGGED signal, which is emitted when a content object's tags have changed.

ISSUE: https://github.com/openedx/modular-learning/issues/197

Dependencies:

• Needed for edx-platform: https://github.com/open-craft/edx-platform/pull/647

Testing instructions:

Follow instructions mentioned in https://github.com/open-craft/edx-platform/pull/647

Reviewers:

• [ ] tag reviewer
• [ ] tag reviewer

Merge checklist:

• [ ] All reviewers approved
• [ ] CI build is green
• [ ] Version bumped
• [ ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed

Post merge:

• [ ] Create a tag
• [ ] Check new version is pushed to PyPI after tag-triggered build is finished.
• [ ] Delete working branch (if not needed anymore)

Author concerns: List any concerns about this PR - inelegant solutions, hacks, quick-and-dirty implementations, concerns about migrations, etc.

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Private-ref: FAL-3691

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[yusuf-musleh]

@mariajgrimaldi Thanks for the review! I've updated the PR addressing the comments.

[mariajgrimaldi]

I left a few questions here: https://github.com/open-craft/edx-platform/pull/647. I'll return once they've answered. Thanks!

[openedx-webhooks]

@yusuf-musleh 🎉 Your pull request was merged! Please take a moment to answer a two question survey so we can improve your experience in the future.

[mariajgrimaldi]

New release: https://github.com/openedx/openedx-events/releases/tag/v9.6.0