<!-- PR description-->
Introducing a new `Configurer` interface to abstract out storage config information(for s3, filesystem etc) from caller code. I consider this as a short term solution. We need to consolidate overall config handling in a better way, but that's out of scope for this PR chain.
Testing
* Most of the changes here are code movement under the hood. So relying on existing tests.
* I'll address any test gaps in a later PR.
---
#### 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
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- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup
#### Issue(s)
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* https://github.com/alcionai/corso/issues/1416
#### Test Plan
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- [x] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E
<!-- PR description-->
One of the 2 remaining setup PRs before we can introduce local storage repos.
**Changes:**
1. Read storage provider(`provider`) from config file except for `repo init * ` or `repo connect *` commands.
2. Apply flag overrides based on provider type( e.g. `S3FlagOverrides` if provider is `S3`)
3. Propagate storage provider type to functions which read/write config. These functions arbitrate on config hierarchy - flags, env, config file, in that order.
**Reasons**
* Reason 1 is needed is because config file is the source of truth for storage provider for all commands except `repo init` or `repo connect`. In the exception cases, we pick the provider in command (e.g. `s3`) as the source of truth. e.g. consider a `repo init s3`, followed by `repo init filesystem`.During `repo init filesystem`, config file would indicate `S3` provider, but the correct behavior here is to select `filesystem` provider.
* One alternative was to push provider from the init/connect cmds into an override flag, and let the config code decide on hierarchy. However, this felt hacky. provider here is not a flag to begin with. It's part of init/connect commands.
---
#### 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: --->
- [ ] 🌻 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. -->
* https://github.com/alcionai/corso/issues/1416
#### Test Plan
<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E
<!-- PR description-->
Follow the pattern used elsewhere for declaring flag names & values. No logic change, just moving code. Added a unit test for flags.
---
#### 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
- [ ] 💚 E2E
