MDEV-11365 Split the data type and attribute related code in Item_sum_hybrid::fix_fields into Type_handler::Item_sum_hybrid_fix_length_and_dec()

This patch:
- Implements the task according to the description
- Adds a new class Type_handler_numeric as a common parent
  for Type_handler_real_result, Type_handler_int_result,
  Type_handler_decimal_result, to share the common code
  between numeric data type handlers.
- Removes the dedundant call for collation.set(item->collation) in
  Item_sum_hybrid::setup_hybrid(), because setup_hybrid() is called
  either after fix_length_and_dec() or afte ther constructor
  Item_sum_hybrid(THD *thd, Item_sum_hybrid *item),
  so the collation is already properly set in all cases.