[ENH] ngboost survival prediction model

[fkiraly]

The PR #215 has added ngboost as an skpro regressor.

The package can also be invoked for survival prediction, i.e., for using censoring information C as in the skpro survival models.
We should also add a version of the ngboost algorithm for survival prediction, hence.

To avoid user confusion, it might be better to add this as a separate estimator in the survival module, although the two estimators may end up sharing a lot of similarities and hence, possibly, an inheritance tree.

[ShreeshaM07]

Should I start working on this issue?

[fkiraly]

If you find it interesting, of course!

Given that you've interfaced ngboost first time round, it should be less work for you than anyone.

[fkiraly]

Perhaps the most useful examples of recently interfaced survival models:

#247
#237

The main additional challenges are:

• handling of the censoring variable - this should include None, and taking care of "which way round" (is 1 censored or uncensored)
• handling of the return distribution - many packages do not create an explicit distribution or an incomplete distribution (see [ENH] design - dealing with incomplete distributions such as predictive survival function estimates #249 ), but this may be easier for ngboost as it uses parametric distributions primarily