<!-- PR description-->
One of the setup PRs before introducing local storage support for corso.
Changes
1. Export `storage.storageProvider` and rename it to `storage.Provider`. Without the rename, linter errors out with `exported: type name will be used as storage.StorageProvider by other packages, and that stutters; consider calling this Provider`
2. Rerun stringer - `stringer -type=Provider -linecomment`
3. Introduce a new Provider - `FS` for filesystem. This is in line with kopia naming for local/network attached storage.
4. Fix hardcoded string values for storage providers in test code.
---
#### 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.-->
- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E
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