Define core multi-target regression tree structure. (#8884)

- Define a new tree struct embedded in the `RegTree`.
- Provide dispatching functions in `RegTree`.
- Fix some c++-17 warnings about the use of nodiscard (currently we disable the warning on
  the CI).
- Use uint32_t instead of size_t for `bst_target_t` as it has a defined size and can be used
  as part of dmlc parameter.
- Hide the `Segment` struct inside the categorical split matrix.

R-package/src/Makevars.win

@@ -60,6 +60,7 @@ OBJECTS= \
     $(PKGROOT)/src/tree/param.o \
     $(PKGROOT)/src/tree/fit_stump.o \
     $(PKGROOT)/src/tree/tree_model.o \
+    $(PKGROOT)/src/tree/multi_target_tree_model.o \
     $(PKGROOT)/src/tree/tree_updater.o \
     $(PKGROOT)/src/tree/updater_approx.o \
     $(PKGROOT)/src/tree/updater_colmaker.o \