Extract optional weight. (#8747)

- Extract optional weight from coommon.h to reduce dependency on this header.
- Add test.

src/common/stats.h

@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "common.h"              // AssertGPUSupport, OptionalWeights
+#include "optional_weight.h"     // OptionalWeights
 #include "transform_iterator.h"  // MakeIndexTransformIter
 #include "xgboost/context.h"     // Context
 #include "xgboost/linalg.h"