<!-- PR description-->
As title
---
#### 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
<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup
#### Issue(s)
<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>
#### Test Plan
<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E
<!-- PR description-->
* Integrates corso extensions into onedrive/sharepoint library item backup flows.
* Also includes integration tests for OD/SP, unit tests for collections.
* Includes a small fix for `rw *backupStreamReader Close()`
Remaining things which will be covered in later PRs:
* extension tests with incremental backups
* Observability related changes for extensions
---
#### Does this PR need a docs update or release note?
- [ ] ✅ Yes, it's included
- [x] 🕐 Yes, but in a later PR
- [ ] ⛔ No
#### Type of change
<!--- Please check the type of change your PR introduces: --->
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* internal
#### Test Plan
<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [x] 💚 E2E
<!-- PR description-->
Introduces interfaces for corso extensions, which is basically wrappers around `io.ReadCloser`. Will add integration in follow up PRs.
---
#### Does this PR need a docs update or release note?
- [ ] ✅ Yes, it's included
- [x] 🕐 Yes, but in a later PR
- [ ] ⛔ No
#### Type of change
<!--- Please check the type of change your PR introduces: --->
- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* #<issue>
#### Test Plan
<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E
