ORAS get-manifest command

[SteveLasker]

ORAS Artifacts, notation and other clis associate references to a digest. However, users think of tags. The oras CLI and go-lib would take a tag, and return a digest, and the associated manifest.

[shizhMSFT]

@FeynmanZhou What do you suggest on the UX of oras get-manifest?

[sajayantony]

I have outlined some details here - https://github.com/oras-project/oras/discussions/340 - @FeynmanZhou

[FeynmanZhou]

I have outlined some details here - #340 - @FeynmanZhou

@sajayantony Thanks for giving more context to this feature request. I assume we still don't have a spec for ORAS manifest commands. I would like to investigate and write a spec for it based on https://github.com/oras-project/oras/discussions/340.

@SteveLasker @shizhMSFT Could you assign this issue to me?

[qweeah]

@shizhMSFT Please assign it to me.

[qweeah]

~~As is described in #459, also need to add --descriptor flag for fetching a descriptor of the target manifest.~~ Update: Moved this into a new issue.