Compatibility mode on oras push

[shizhMSFT]

What is the version of your ORAS CLI

0.16.0

What would you like to be added?

Add a compatibility mode for oras push so that it can force pushing OCI image manifest instead of OCI artifact manifest.

Why is this needed for ORAS?

The state-of-the-art registries supports OCI artifact manifest. Since 0.16.0, oras push pushes OCI artifact manifest to those registries which support it. However, the new manifest type may not be supported at the consumer side (e.g. self-crafted scripts). It would be better if we can provide a compatibility mode for smoother migration from OCI image manifests to OCI artifact manifests.

The current workaround for enabling the combability mode is to specify a --config flag. Since OCI artifact manifest does not have a config, it will push an OCI image manifest instead. However, it is not user-friendly and is a bit hacky. We should provide a clearer way to do so.

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

• Yes, I am willing to implement it.

[shizhMSFT]

Resolved by #741