Merge features in artifacts branch as experimental features

[shizhMSFT]

With oras-go v2, we can "merge" (re-implement) those features in the artifacts branch into main with those features marked as experimental.

Features:

• oras discover.
• oras push artifact manifest.
• oras pull artifact manifest.

[shizhMSFT]

@qweeah You may create issues according to this issue.

[qweeah]

ack

[qweeah]

Implementation Gaps

Pull

This command already exists in the main branch. Below options are new from the artifacts branch:

• media-type and allow-all: Since we have already removed the metia-type filtering features in oras pull shouldn't require --media-type or --allow-all #130, those options are deprecated.
• allow-empty-name: File target already support in-memory caching for non-named manifests.

Basically there is no extra code needed for the pull command.

Discover

This is a new command, we need to support discovering artifacts linked with a specified reference, with optional artifact type filtering and different formats of output.

Push

This command already exists in the main branch. Below options are new from the artifacts branch:

• export-manifest: User will use this to save the packed manifest to a local file. We will refetch the manifest from the source file target and write to a file. It would be better if export manifest can be supported native by oras-go manifest packing options.

• subject and artifact-type: those two options will be removed and re-implemented in Specification for ORAS attach command #424

[shizhMSFT]

Closed by #419 and #425