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Message handler implementation and other APIs to fetch channels and messages data
#### Does this PR need a docs update or release note?
- [ ] ⛔ No
#### Type of change
<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
#### Issue(s)
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* #<issue>
#### Test Plan
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- [ ] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E
Adds the boilerplate for groups backup collection processing. Not necessarily functional at this time, due to missing dependencies and consts that aren't yet in the branch. Thus the lack of tests. It's just good enough to keep progress rolling forward.
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#### Does this PR need a docs update or release note?
- [x] ⛔ No
#### Type of change
- [x] 🌻 Feature
#### Issue(s)
* #3989
#### Test Plan
