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.

Start by reviewing the package's existing KM-estimate reference-line support and the survfit entry point, including its cumhaz output. Compare the desired cumulative, hazard, and log-hazard estimates, then investigate muhaz for grouped hazard calculations and define the supported behavior before implementation.

Escrito por el modelo de indexación a partir del texto del issue.