registry: implement fetchManifest in OciRegistryClient (salesforce#295)

Author: iamabhilaksh

registry/registry-client/src/main/java/com/salesforce/multicloudj/registry/driver/Manifest.java

@@ -0,0 +1,44 @@
+package com.salesforce.multicloudj.registry.driver;
+
+/**
+ * OCI Image Manifest JSON field names as defined in the OCI Image Specification.
+ * 
+ * @see <a href="https://github.com/opencontainers/image-spec/blob/v1.0.1/manifest.md">OCI Image Manifest Specification</a>
+ * @see <a href="https://github.com/opencontainers/image-spec/blob/v1.0.1/image-index.md">OCI Image Index Specification</a>
+ */
+final class OciManifestFields {
+
+    private OciManifestFields() {
+        // Utility class - prevent instantiation
+    }
+
+    // OCI media types
+    static final String MEDIA_TYPE_OCI_INDEX = "application/vnd.oci.image.index.v1+json";
+    static final String MEDIA_TYPE_OCI_MANIFEST = "application/vnd.oci.image.manifest.v1+json";
+    static final String MEDIA_TYPE_DOCKER_MANIFEST_LIST = "application/vnd.docker.distribution.manifest.list.v2+json";
+    static final String MEDIA_TYPE_DOCKER_MANIFEST = "application/vnd.docker.distribution.manifest.v2+json";
+
+    // Common descriptor fields
+    static final String MEDIA_TYPE = "mediaType";
+    static final String DIGEST = "digest";
+    static final String SIZE = "size";
+
+    // Image manifest fields
+    static final String CONFIG = "config";
+    static final String LAYERS = "layers";
+
+    // Image index fields
+    static final String MANIFESTS = "manifests";
+
+    // Optional fields (manifest and index)
+    static final String ANNOTATIONS = "annotations";
+    static final String SUBJECT = "subject";
+
+    // Platform fields (in image index manifest entries)
+    static final String PLATFORM = "platform";
+    static final String OS = "os";
+    static final String ARCHITECTURE = "architecture";
+    static final String OS_VERSION = "os.version";
+    static final String OS_FEATURES = "os.features";
+    static final String VARIANT = "variant";
+}