Support modelpack/model-spec file path

[bitoku]

In the model-spec, org.cncf.model.filepath annotation is supposed to be used as a file path when mounting.

https://github.com/modelpack/model-spec/blob/e262ddc3d2bf8036b81de7768a5edfbe67045f4e/specs-go/v1/annotations.go#L22-L23

However libartifact can only use org.opencontainers.image.title as a file path.

container-libs/common/pkg/libartifact/store/store.go

Lines 440 to 495 in c780818

	// BlobMountPaths allows the caller to access the file names from the store and how they should be mounted.
	func (as ArtifactStore) BlobMountPaths(ctx context.Context, nameOrDigest string, options *libartTypes.BlobMountPathOptions) ([]libartTypes.BlobMountPath, error) {
	arty, imgSrc, err := getArtifactAndImageSource(ctx, as, nameOrDigest, &options.FilterBlobOptions)
	if err != nil {
	return nil, err
	}
	defer imgSrc.Close()
	if len(options.Digest) > 0 || len(options.Title) > 0 {
	digest, err := findDigest(arty, &options.FilterBlobOptions)
	if err != nil {
	return nil, err
	}
	// In case the digest is set we always use it as target name
	// so we do not have to get the actual title annotation form the blob.
	// Passing options.Title is enough because we know it is empty when digest
	// is set as we only allow either one.
	filename, err := generateArtifactBlobName(options.Title, digest)
	if err != nil {
	return nil, err
	}
	path, err := layout.GetLocalBlobPath(ctx, imgSrc, digest)
	if err != nil {
	return nil, err
	}
	return []libartTypes.BlobMountPath{{
	SourcePath: path,
	Name: filename,
	}}, nil
	}
	mountPaths := make([]libartTypes.BlobMountPath, 0, len(arty.Manifest.Layers))
	for _, l := range arty.Manifest.Layers {
	title := l.Annotations[specV1.AnnotationTitle]
	for _, mp := range mountPaths {
	if title == mp.Name {
	return nil, fmt.Errorf("annotation %q:%q is used in multiple different layers within artifact", specV1.AnnotationTitle, title)
	}
	}
	filename, err := generateArtifactBlobName(title, l.Digest)
	if err != nil {
	return nil, err
	}
	path, err := layout.GetLocalBlobPath(ctx, imgSrc, l.Digest)
	if err != nil {
	return nil, err
	}
	mountPaths = append(mountPaths, libartTypes.BlobMountPath{
	SourcePath: path,
	Name: filename,
	})
	}
	return mountPaths, nil
	}

I'm not sure what is the best way though.
Off the top of my head, BlobMountPaths could accept file path annotation as an argument, or we could introduce a new option for filepath.
Do you have any plan to use org.cncf.model.filepath as a file path?

The past discussion regarding the file path annotation: modelpack/model-spec#58