extend_schema: preserve "description" and "extensions"

Replicates graphql/graphql-js@6b5c8af

src/graphql/utilities/extend_schema.py

@@ -259,10 +259,13 @@ def extend_schema_args(
                 for directive in schema_kwargs["directives"]
             )
             + tuple(self.build_directive(directive) for directive in directive_defs),
-            description=schema_def.description.value
-            if schema_def and schema_def.description
-            else None,
-            extensions={},
+            description=(
+                schema_def.description.value
+                if schema_def and schema_def.description
+                else None
+            )
+            or schema_kwargs["description"],
+            extensions=schema_kwargs["extensions"],
             ast_node=schema_def or schema_kwargs["ast_node"],
             extension_ast_nodes=schema_kwargs["extension_ast_nodes"]
             + tuple(schema_extensions),

tests/utilities/test_extend_schema.py

@@ -135,6 +135,16 @@ def does_not_modify_built_in_types_and_directives():
 
         assert extended_schema.directives == specified_directives
 
+    def preserves_original_schema_config():
+        description = "A schema description"
+        extensions = {"foo": "bar"}
+        schema = GraphQLSchema(description=description, extensions=extensions)
+
+        extended_schema = extend_schema(schema, parse("scalar Bar"))
+
+        assert extended_schema.description == description
+        assert extended_schema.extensions is extensions
+
     def extends_objects_by_adding_new_fields():
         schema = build_schema(
             '''