[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/learner.cc

@@ -436,7 +436,7 @@ class LearnerConfiguration : public Learner {
     }
   }
 
-  uint32_t GetNumFeature() override {
+  uint32_t GetNumFeature() const override {
     return learner_model_param_.num_feature;
   }