Add Kopia s3 and s3 compliant storage flags

[plar]

Change Overview

This PR is the 7th in the series of PRs that will add a new way to build Kopia CLI commands.

PR Train:

1. PR-2653 Introducing a new approach to building Kopia CLI commands using safecli v0.0.6
2. PR-2654 Implement Kopia common args and opts
3. PR-2655 Implement Kopia storage helpers
4. PR-2656 Add Kopia Filesystem storage opts
5. PR-2657 Add Kopia GCS storage opts
6. PR-2658 Add Kopia Azure storage opts
7. PR-2659 Add Kopia S3 and S3 compliant storage opts
8. PR-2661 Add Kopia repository create command
9. PR-2662 Add Kopia repository connect command
10. PR-2663 Add Kopia repository connect server command
11. PR-2664 Add Kopia repository set-parameters command
12. PR-2665 Add Kopia repository status command

This PR introduces storage s3 and s3 compliant flag structures which will be used to build storage flags for s3 and s3 compliant storages.

Pull request type

Please check the type of change your PR introduces:

• 🚧 Work in Progress
• 🌈 Refactoring (no functional changes, no api changes)
• 🐹 Trivial/Minor
• 🐛 Bugfix
• 🌻 Feature
• 🗺️ Documentation
• 🤖 Test

Test Plan

• 💪 Manual
• ⚡ Unit test
• 💚 E2E

[kale-amruta]

Is ArgTest a utilitity to be used across different packages. This doesnt seem relevant to s3 package. Should we move it to a different package or utils package?

[plar]

Agreed. I will move it to pkg/kopia/cli/internal/test

[kale-amruta]

left few comments

[kale-amruta]

lgtm