Add support for other survival estimates

Currently using this package it is only possible to add reference lines for survival estimates via KM estimates. However it is common for users to plot their survival data in terms of transformations of the survival function e.g.

- Hazard function
- Cumulative Hazard function
- Log Hazard function

It would be good for this package to support these as well.

Cumulative Hazard estimates are already available from the `survfit` function
```
x <- survfit(Surv(time, event) ~ 1, data = dat)
x$cumhaz
```

Hazard functions are more complex. I'm not sure what the best way is but I see many recommendations for [`muhaz`](https://rdrr.io/cran/muhaz/man/muhaz.html) e.g.
```
muhaz::muhaz(dat$time, dat$event)
```
Though it doesn't appear to have a grouping option so you would need to loop over the required groups.

Commencez par examiner la prise en charge existante par le package des lignes de référence de KM-estimate et du point d’entrée survfit, y compris sa sortie cumhaz. Comparez les estimations cumulées, de hazard et de log-hazard souhaitées, puis étudiez muhaz pour les calculs de hazard groupés et définissez le comportement pris en charge avant l’implémentation.

Rédigé par le modèle d'indexation à partir du texte de l'issue.