Add column showing labels in update report

[oyvindeide]

Related to: https://github.com/equinor/ert/issues/6262

Would like to add a new column, for summary data the value in the column would be the datetime of the observation, for gen_data it would be report step and index. Would like this to be contained within _es_update, and probably just add a new property to: https://github.com/equinor/ert/blob/b95fea52dae0455759d8d7073f612d0e10a0264c/src/ert/analysis/_es_update.py#L264, which would just be a string of index