Define feature mark behavior for [Experimental] and [Preview]

[FeynmanZhou]

What is the version of your ORAS CLI

v1.0.0

What would you like to be added?

ORAS has introduced three feature marks to divide the CLI command feature maturity into three stages:

• Experimental: This feature can radically change or completely disappear at any time. It may be in development but for the early trial. It's not recommended to use it in production; It may introduce breaking changes in the future. It requires setting an environmental variable to enable the experimental feature. It should have a warning message to remind users. Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.

• Preview: This feature development is almost done and is closing to stable. It is visible to all users by default with [Preview] feature mark and can be used without any additional settings. But it's recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases.

• Stable: This feature is mature and should be always enabled; The corresponding feature mark is no longer needed. Stable versions of features will appear in released software for many subsequent versions.

Why is this needed for ORAS?

Provide flexibility for new features that are still in progress and gather feedback from early adopters. Popular project like Kubernetes has its feature gate as a reference for users: https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/#feature-stages

Are you willing to submit PRs to contribute to this feature?

• Yes, I am willing to implement it.

[qweeah]

Sounds good! Should also emphasize that the preview feature is still subject to change. Also need to document this somehow.

[FeynmanZhou]

@qweeah Yes. we need to write a document to explain these two feature marks.