feat: return modified field on get component endpoint

Author: rpenido

openedx/core/djangoapps/content_libraries/serializers.py

@@ -154,6 +154,7 @@ class LibraryXBlockMetadataSerializer(serializers.Serializer):
     last_draft_created_by = serializers.CharField(read_only=True)
     has_unpublished_changes = serializers.BooleanField(read_only=True)
     created = serializers.DateTimeField(format=DATETIME_FORMAT, read_only=True)
+    modified = serializers.DateTimeField(format=DATETIME_FORMAT, read_only=True)
 
     # When creating a new XBlock in a library, the slug becomes the ID part of
     # the definition key and usage key: