osbuild-mpp: extract is_manifest_list() function

Extract the is_manifest_list() function from the ImageManifest object in
osbuild-mpp into a util function to be reused by the skopeo source.

osbuild/util/containers.py

@@ -0,0 +1,14 @@
+def is_manifest_list(data):
+    """Inspect a manifest determine if it's a multi-image manifest-list."""
+    media_type = data.get("mediaType")
+    #  Check if mediaType is set according to docker or oci specifications
+    if media_type in ("application/vnd.docker.distribution.manifest.list.v2+json",
+                      "application/vnd.oci.image.index.v1+json"):
+        return True
+
+    # According to the OCI spec, setting mediaType is not mandatory. So, if it is not set at all, check for the
+    # existence of manifests
+    if media_type is None and data.get("manifests") is not None:
+        return True
+
+    return False