Support named mapping for task group

### Description Currently, named mapping for dynamic tasks is supported through the map_index_template property. However, this feature is not available for task groups. We would be happy to help with this issue, but we might need some guidance from the project's mentors. ### Use case/motivation We would like to have labels instead of index on all tasks within the dynamic task group ### Related issues _No response_ ### Are you willing to submit a PR? - [ ] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)