[dask] Add DaskXGBRanker (#6576)

* Initial support for distributed LTR using dask.

* Support `qid` in libxgboost.
* Refactor `predict` and `n_features_in_`, `best_[score/iteration/ntree_limit]`
  to avoid duplicated code.
* Define `DaskXGBRanker`.

The dask ranker doesn't support group structure, instead it uses query id and
convert to group ptr internally.

src/c_api/c_api.cc

@@ -498,6 +498,7 @@ XGB_DLL int XGBoosterGetNumFeature(BoosterHandle handle,
                                    xgboost::bst_ulong *out) {
   API_BEGIN();
   CHECK_HANDLE();
+  static_cast<Learner*>(handle)->Configure();
   *out = static_cast<Learner*>(handle)->GetNumFeature();
   API_END();
 }