feat: Support pulling arbitrary manifest config

[lizMSFT]

#274 enables the CLI to pull manifest config. However, the user must provide the desired media type of the config or use the default. This PR allows pulling arbitrary manifest config.

If the user does not specify the desired media type, the code will get the 0th indexed node as the manifest config if its parent node is a manifest.
Note: For a manifest, the 0th indexed element is always a manifest config. ref: https://github.com/oras-project/oras-go/blob/6a09a65fcc0b96c3448e988c1727ed154c2388ea/content/graph.go#L58

Examples:

$ oras pull --manifest-config config.json localhost:5000/hello:latest
Downloaded  3ad08644ab5a artifact.txt
Downloaded  98df5c495df6 config.json
Pulled localhost:5000/hello:latest
Digest: sha256:8627dfe823698a566e105d961045eafcaa9115b6faf1f625edd85df558421bd5

Resolves: #275
Signed-off-by: Zoey Li zoeyli@microsoft.com

[qweeah]

Verified on my side and functionally looks good.

[qweeah]

@shizhMSFT Do we need to print warning if: no config media type is specified and there is already a name(title annotation) in the config descriptor?

[sajayantony]

Recommend simplifying this to to just oras pull --config. I don't believe there is any other object in OCI that is called config.

[codecov-commenter]

Codecov Report

Merging #480 (e098299) into main (7c6fbdb) will not change coverage.
The diff coverage is n/a.

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  Misses         90       90           
  Partials       15       15           

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[shizhMSFT]

--config is already reserved for auth config.

See

  -c, --config stringArray       auth config path

[shizhMSFT]

@lizMSFT @sajayantony @qweeah There is another option that we can rename --config to --auth-config and use --config for --manifest-config.

[qweeah]

@lizMSFT @sajayantony @qweeah There is another option that we can rename --config to --auth-config and use --config for --manifest-config.

How about --cred-file? Previously we used --config to align with the naming of docker config but it's not easy to understand out of the docker context.

We can still use -c for short.

[sajayantony]

This is tricky. If you have a config file that’s for auth and config file for manifest.
Im ok with manifest config or auth config. since —config /auth-config is a global flag and if it was there before then we might not want to break previous non alpha - releases.
Basically you folks decide. 😁

[lizMSFT]

Yes, I am feeling the same. It would be better not change the name for now since it will bring breaking changes and it currently doesn't bring additional value. Besides, consumers will need to adjust accordingly, which might make them confused.

[lizMSFT]

Create a new issue for rename --manifest-config: #495
cc @shizhMSFT @qweeah @sajayantony

[shizhMSFT]

@sajayantony Let's continue the discussion in #495.

[shizhMSFT]

@shizhMSFT Do we need to print warning if: no config media type is specified and there is already a name(title annotation) in the config descriptor?

No, we don't.

[shizhMSFT]

LGTM