Add withArtifactType for Index (#590)

Signed-off-by: Valentin Delaye <jonesbusy@users.noreply.github.com>

Author: jonesbusy

src/main/java/land/oras/Index.java

@@ -205,6 +205,16 @@ public ManifestDescriptor getDescriptor() {
         return null;
     }
 
+    /**
+     * Return a new index with the given artifact type
+     * @param artifactType The artifact type
+     * @return The index
+     */
+    public Index withArtifactType(String artifactType) {
+        return new Index(
+                schemaVersion, mediaType, artifactType, manifests, annotations, subject, descriptor, registry, json);
+    }
+
     /**
      * Return a new index with the given descriptor
      * @param descriptor The descriptor

src/test/java/land/oras/IndexTest.java

@@ -55,6 +55,18 @@ void shouldReadAndWriteIndex() {
                 result);
     }
 
+    @Test
+    void shouldAddArtifactType() {
+        Index index = Index.fromManifests(List.of());
+        index = index.withArtifactType("application/vnd.opentofu.provider");
+        assertNotNull(index.getArtifactType());
+        assertEquals(
+                "application/vnd.opentofu.provider", index.getArtifactType().getMediaType());
+        assertEquals(
+                "{\"schemaVersion\":2,\"mediaType\":\"application/vnd.oci.image.index.v1+json\",\"artifactType\":\"application/vnd.opentofu.provider\",\"manifests\":[]}",
+                index.toJson());
+    }
+
     @Test
     void shouldReadAndWriteIndexWithAnnotations() {
         String json =