Implement a way to search for an image by manifest, config or layer digest

```
Usage:
  zot images [config-name] [flags]

Flags:
  -d, --digest string   List images containing a specific manifest, config, or layer digest
[...]
```

pkg/extensions/search/common/oci_layout.go

@@ -0,0 +1,152 @@
+// Package common ...
+package common
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path"
+	"strings"
+
+	"github.com/anuvu/zot/errors"
+	"github.com/anuvu/zot/pkg/log"
+	"github.com/anuvu/zot/pkg/storage"
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	godigest "github.com/opencontainers/go-digest"
+	ispec "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+// CveInfo ...
+type OciLayoutUtils struct {
+	Log             log.Logger
+	StoreController storage.StoreController
+}
+
+// NewOciLayoutUtils initializes a new OciLayoutUtils object.
+func NewOciLayoutUtils(storeController storage.StoreController, log log.Logger) *OciLayoutUtils {
+	return &OciLayoutUtils{Log: log, StoreController: storeController}
+}
+
+// Below method will return image path including root dir, root dir is determined by splitting.
+func (olu OciLayoutUtils) GetImageRepoPath(image string) string {
+	var rootDir string
+
+	prefixName := GetRoutePrefix(image)
+
+	subStore := olu.StoreController.SubStore
+
+	if subStore != nil {
+		imgStore, ok := olu.StoreController.SubStore[prefixName]
+		if ok {
+			rootDir = imgStore.RootDir()
+		} else {
+			rootDir = olu.StoreController.DefaultStore.RootDir()
+		}
+	} else {
+		rootDir = olu.StoreController.DefaultStore.RootDir()
+	}
+
+	return path.Join(rootDir, image)
+}
+
+func (olu OciLayoutUtils) GetImageManifests(imagePath string) ([]ispec.Descriptor, error) {
+	buf, err := ioutil.ReadFile(path.Join(imagePath, "index.json"))
+
+	if err != nil {
+		if os.IsNotExist(err) {
+			olu.Log.Error().Err(err).Msg("index.json doesn't exist")
+
+			return nil, errors.ErrRepoNotFound
+		}
+
+		olu.Log.Error().Err(err).Msg("unable to open index.json")
+
+		return nil, errors.ErrRepoNotFound
+	}
+
+	var index ispec.Index
+
+	if err := json.Unmarshal(buf, &index); err != nil {
+		olu.Log.Error().Err(err).Str("dir", imagePath).Msg("invalid JSON")
+		return nil, errors.ErrRepoNotFound
+	}
+
+	return index.Manifests, nil
+}
+
+func (olu OciLayoutUtils) GetImageBlobManifest(imageDir string, digest godigest.Digest) (v1.Manifest, error) {
+	var blobIndex v1.Manifest
+
+	blobBuf, err := ioutil.ReadFile(path.Join(imageDir, "blobs", digest.Algorithm().String(), digest.Encoded()))
+	if err != nil {
+		olu.Log.Error().Err(err).Msg("unable to open image metadata file")
+
+		return blobIndex, err
+	}
+
+	if err := json.Unmarshal(blobBuf, &blobIndex); err != nil {
+		olu.Log.Error().Err(err).Msg("unable to marshal blob index")
+
+		return blobIndex, err
+	}
+
+	return blobIndex, nil
+}
+
+func (olu OciLayoutUtils) GetImageInfo(imageDir string, hash v1.Hash) (ispec.Image, error) {
+	var imageInfo ispec.Image
+
+	blobBuf, err := ioutil.ReadFile(path.Join(imageDir, "blobs", hash.Algorithm, hash.Hex))
+	if err != nil {
+		olu.Log.Error().Err(err).Msg("unable to open image layers file")
+
+		return imageInfo, err
+	}
+
+	if err := json.Unmarshal(blobBuf, &imageInfo); err != nil {
+		olu.Log.Error().Err(err).Msg("unable to marshal blob index")
+
+		return imageInfo, err
+	}
+
+	return imageInfo, err
+}
+
+func GetRoutePrefix(name string) string {
+	names := strings.SplitN(name, "/", 2)
+
+	if len(names) != 2 { // nolint: gomnd
+		// it means route is of global storage e.g "centos:latest"
+		if len(names) == 1 {
+			return "/"
+		}
+	}
+
+	return fmt.Sprintf("/%s", names[0])
+}
+
+func DirExists(d string) bool {
+	fi, err := os.Stat(d)
+	if err != nil && os.IsNotExist(err) {
+		return false
+	}
+
+	return fi.IsDir()
+}
+
+func GetImageDirAndTag(imageName string) (string, string) {
+	var imageDir string
+
+	var imageTag string
+
+	if strings.Contains(imageName, ":") {
+		splitImageName := strings.Split(imageName, ":")
+		imageDir = splitImageName[0]
+		imageTag = splitImageName[1]
+	} else {
+		imageDir = imageName
+	}
+
+	return imageDir, imageTag
+}