Implement Kopia storage helpers

[plar]

Change Overview

This PR is the 3rd 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 core storage flag structures and storage helpers which will be used to build storage flags for specific providers(filesystem, s3, etc), along with extended error definitions.

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

[PrasadG193]

Shouldn't we handle error?

[plar]

I've implemented the same logic as it is in the current implementation:

// pkg/kopia/command/storage/secret_utils.go
func checkSkipSSLVerifyFromMap(m map[string][]byte) bool {
	v := string(m[repositoryserver.SkipSSLVerifyKey])
	return v == "true"
}

strconv.ParseBool returns false in the case of error.

[PrasadG193]

Why do we need this?

[plar]

This Nop logger is used inside s3 storage as a default logger if no logger is provided.
It helps to avoid code cluttering and remove some code bumps.

You don't need to write something like below:

    if logger != nil {
        logger.Print("Removing trailing slashes from the endpoint")
    }

and can simply use:

    logger.Print("Removing trailing slashes from the endpoint")

[PrasadG193]

Is trailing / compulsory? What's the problem if we don't add?

[plar]

This code is copied from pkg/kopia/command/storage/utils.go. @ankitjain235, can you help us understand why a trailing slash is needed here?

[ankitjain235]

I think it's needed, but will let @pavannd1 confirm. Anyways it's existing behaviour so should be good to merge, we can remove it if not required later?