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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