Add metadata annotations for generated Python protobuf symbols.

The pyi generator now includes Kythe annotations for:
*   Extension field constants (e.g., `EXTENSION_FIELD`).
*   Field number constants (e.g., `STRING_FIELD_FIELD_NUMBER`).
*   `Create` methods in generated Stubby client classes.

These annotations allow the Python indexer to link these generated symbols back to their definitions in the `.proto` files.

PiperOrigin-RevId: 852971617