feat: Add Waffle config watcher

[timmc-edx]

Listens to Waffle model changes and reports them 1) to the log and 2) to a Slack webhook.

This adds a dependency on django-waffle.

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To test this in devstack:

• Check out the branch under src, e.g. git worktree add ../src/edxarch-waffle/ timmc/waffle-watcher
• In lms-shell: pip install -e /edx/src/edxarch-waffle/
• In edx-platform's lms/envs/common.py, add 'edx_arch_experiments.config_watcher' to INSTALLED_APPS
• Log in to LMS as the edx user
• Change, add, or delete a Waffle flag, switch, or sample
• Watch for the log message

You can also configure CONFIG_WATCHER_SLACK_WEBHOOK_URL to a request bin to confirm the request format.

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Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets