Add class Item_sum_min_max

Under terms of this task we'll do the following:

• Rename the old Item_sum_hybrid to Item_sum_min_max
• Add a new Item_sum_hybrid like this:

  class Item_sum_hybrid: public Item_sum,

  public Type_handler_hybrid_field_type

  {

  public:

  Item_sum_hybrid(THD *thd, Item *item_par):

  Item_sum(thd, item_par),

  Type_handler_hybrid_field_type(&type_handler_longlong)

  { collation.set(&my_charset_bin); }

  Item_sum_hybrid(THD *thd, Item *a, Item *b):

  Item_sum(thd, a, b),

  Type_handler_hybrid_field_type(&type_handler_longlong)

  { collation.set(&my_charset_bin); }

  Item_sum_hybrid(THD *thd, Item_sum_hybrid *item)

  :Item_sum(thd, item),

  Type_handler_hybrid_field_type(item)

  { }

  const Type_handler *type_handler() const

  { return Type_handler_hybrid_field_type::type_handler(); }

  };

• Derive Item_sum_min_max from the new Item_sum_hybrid
• Reuse the new Item_sum_hybrid in Item_sum_hybrid_simple as follows:

  -class Item_sum_hybrid_simple : public Item_sum,

  - public Type_handler_hybrid_field_type

  +class Item_sum_hybrid_simple : public Item_sum_hybrid

Later, when we fix MDEV-20272, we'll also change Item_sum_percentile_disc (and probably Item_sum_percentile_cont) to be descendants of the new Item_sum_hybrid rather than Item_sum_num.

This expression:

PERCENTILE_DISC(0.5) WITHIN GROUP (ORDER BY time_column)

should return a TIME value. Therefore the underlying class cannot be a descentand of Item_sum_num (which is supposed to be of a numeric type only).