Support to customize the config data

[chlins]

What is the version of your ORAS CLI

v1.1.0

What would you like to be added?

Support to customize the config descriptor such as mediaType when using ORAS to attach something to a subject artifact.

Why is this needed for ORAS?

OCI does not require mandatory content for the config descriptor, so in some scenarios, users may need to define their own artifacts, which may need to customize the type of their config.

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

• [ ] Yes, I am willing to implement it.

[qweeah]

This is already supported by oras push --config <file>[:<type>] but still missing in oras attach. One valid use case is Artifact with config data (#3 in image-spec guidance).