feat: support oci layout target

[qweeah]

This PR added a new flag --oci to support oci layout in all commands EXCEPT below:

• oras manifest delete, oras blob delete: oras-go doesn't support Garbage Collection for OCI targets.
• oras discover: OCI layout referrer is not well designed in oras-go yet and graph predecessors won't contain artifact type.
• oras login, oras logout and oras repo ls: OCI layout itself represents a local repository, not a remote registry and won't need to support those.

The reference format of a OCI layout reference is <path>[:<tag>|@<digest>]
You can:

1. Pull an image in oci layout to local folder ghcr.io/oras-project/oras with tag v0.16.0

oras cp ghcr.io/oras-project/oras:v0.16.0 ghcr.io/oras-project/oras:v0.16.0 --to-oci

you can inspect the pulled image via

oras manifest fetch  ghcr.io/oras-project/oras:v0.16.0 --oci
oras manifest fetch  ghcr.io/oras-project/oras:v0.16.0 --platform linux/amd64 --oci #inspect certain platform

2. Push the image into a remote repository oci-layout in registry some.registry with tag from-folder, you can simply do

$IMAGE="some.registry/oci-layout:from-folder"
oras cp ghcr.io/oras-project/oras:v0.16.0 $IMAGE --from-oci 

or push from a tarball

cd ghcr.io/oras-project/oras
tar -cvf pulled.tar *
oras cp pulled.tar:test $IMAGE --from-oci 

3. Attach a signature towards it

echo sig > sig
oras attach ghcr.io/oras-project/oras:v0.16.0 sig --artifact-type test.signature --oci

[codecov-commenter]

Codecov Report

Merging #692 (e161136) into main (09e997a) will decrease coverage by 9.94%.
The diff coverage is 18.54%.

@@            Coverage Diff             @@
##             main     #692      +/-   ##
==========================================
- Coverage   73.17%   63.22%   -9.95%     
==========================================
  Files          16       18       +2     
  Lines         574      688     +114     
==========================================
+ Hits          420      435      +15     
- Misses        123      220      +97     
- Partials       31       33       +2     

Impacted Files	Coverage Δ
cmd/oras/internal/option/parser.go	0.00% <0.00%> (ø)
cmd/oras/internal/option/target.go	11.76% <11.76%> (ø)
cmd/oras/internal/option/platform.go	100.00% <100.00%> (ø)
cmd/oras/internal/option/remote.go	69.51% <100.00%> (+1.21%)	⬆️

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[TerryHowe]

I'm not a big fan of this target parameter. It seems like the type could be encoded in the source and destination like URIs. Create a factory method like you that creates a destination and sources based on the string. E.g.: oras cp file://image.tgz oci://harbor.mine.demo:30003/hello:latest

The default could be an oci registry so oras cp file://image.tgz harbor.mine.demo:30003/hello:latest

[qweeah]

Thanks @TerryHowe for reviewing this. Indeed, the OCI layout type could be encoded in the source and destination like URIs(and it's really a good user experience), but there are still other configurable options that requires user input:

1. what is the source tag/digest for copying?
   The example scenario you are giving should be copying from a local oci layout file image.tgz towards a remote registry harbor.mine.demo:30003/hello with tag latest. We can use oci:// to specify the source type and ignore http(s):// in the dest, so the command looks like below

oras cp oci://image.tgz harbor.mine.demo:30003/hello:latest

In the command, only the path of an oci-layout is provided but what should be copied from image.tgz is not specified, either we create a new flag for that or we use a delimitor so user can specify both path and org.opencontainers.image.ref.name at the same time.

2. what is the type of oci-layout, is it a tarball or folder?
   ORAS is able to speculate those if an oci layout target is the copy source, but what if the copy destination is an oci-layout?
   Say thethe command is: oras cp harbor.mine.demo:30003/hello:latest oci://image. Without dedicated flags,
   the user can possibly want pull harbor.mine.demo:30003/hello:latest and add it to 1) a local oci layout folder named image; 2) a local oci layout tarball named image;

[qweeah]

BTW I am still working on the design spec for supporting oci layout, will share it to you if once it's ready for review

[qweeah]

Will separate changes to smaller PRs

[qweeah]

close as it's merged