text: Title: merge_schemas() does not add the "@stitch" directive definition to the merged schema AST
  Body: When merging two or more schemas with `merge_schemas()` (in `schema_transformation.merge_schemas.py`), the generated `MergedSchemaDescriptor` object contains a `schema_ast` field with an AST for the merged schema. This AST contains uses of the `@stitch` directive, but no definition for that directive – and is therefore invalid.