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)

Start by tracing the existing dynamic-task support for the map_index_template property, then compare it with task-group handling. The work is done when named mapping is supported for task groups so every task uses the requested labels instead of numeric indexes; the issue does not name specific files or tests.

Written by the indexing model from the issue text.