Add `get_surv_times_df()`

### Feature description

This function will handle creation of df containing survival times data, which will be passed to `tbl_survfit_times()`.
Users will be able to modify the table before plotting it or annotating it to a plot created with `gg_km()`.

This follows the same desing idea as was described here for `get_cox_pairwise_df()` https://github.com/insightsengineering/crane/issues/207

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