Get the last time a model was modified and return it in BaseModel. This will help with discovering what items can be garbage collected during incomplete backup cleanup as we don't want to accidentally delete in-flight backups.
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#### Does this PR need a docs update or release note?
- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No
#### Type of change
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
* #3217
#### Test Plan
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E