support --platform in oras push and oras attach

[qweeah]

What is the version of your ORAS CLI

oras v1.1.0-rc.1

What would you like to be added?

Add a new flag --platform to allow baking platform information into config data like

oras push localhost:5000/oras:v1.0.1 --artifact-type "application/vnd.example+type" --platform linux/amd64 oras--1.0.1.arm64_monterey.bottle.tar.gz
oras attach localhost:5000/oras:v1.0.1 --artifact-type "application/vnd.sbom" --platform linux/amd64 oras--1.0.1.arm64_monterey.bottle.tar.gz

The specified platform information will be stored in the config blob like https://oras.land/docs/concepts/artifact#artifacts-with-config

Why is this needed for ORAS?

User can easily build artifact with platform information without composing config blob on their own.

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

• Yes, I am willing to implement it.

[amands98]

Should I include support for a platform flag ? // @qweeah

[qweeah]

@amands98 Thanks for volunteering! Please make sure the baked config is following https://github.com/opencontainers/image-spec/blob/main/config.md#properties

[shizhMSFT]

What's the media type of the config? Is it artifact-type or the standard config type? How is it supported by multiple cloud providers?