This is the first in a series of PRs to get v0 backups working for channels. In this change, the current api enumerators get plugged into the collections handler to produce backup data. Follow-up PRs will:
* hook backup to the CLI
* swap full-item enumeration for id-first-get-later pattern
* populate each message with all its replies on the get-later
* turn on integration testing at the operations and ci layers
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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
- [x] ⚡ Unit test
- [x] 💚 E2E
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