Add class Load_data_outvar and avoid using Item::STRING_ITEM for Item_user_var_as_out_param detection

This task is a self-contained change which is going to simplify MDEV-12927 and MDEV-14630.

Under terms of MDEV-14630 we're going to replace data-type dependent constants STRING_ITEM, INT_ITEM, REAL_ITEM, DECIMAL_ITEM, DATE_ITEM to an universal constant LITERAL_ITEM.

In order to do it easier, we'll clean up the code in sql_load.cc not to use Item::STRING_ITEM for Item_user_var_as_out_param detection.

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

• Introduce a new class:

  class Load_data_outvar

  {

  public:

  virtual ~Load_data_outvar() {}

  virtual bool load_data_set_null(THD *thd, const Load_data_param *param)= 0;

  virtual bool load_data_set_value(THD *thd, const char *pos, uint length,

  const Load_data_param *param)= 0;

  virtual bool load_data_set_no_data(THD *thd, const Load_data_param *param)= 0

  virtual void load_data_print_for_log_event(THD *thd, class String *to) const=0;

  virtual bool load_data_add_outvar(THD *thd, Load_data_param *param) const= 0;

  virtual uint load_data_field_length() const= 0;

  };

• Derive Item_field and Item_user_var_as_out_param from Load_data_outvar.
• Move Item_field- and Item_user_var_as_out_param specific pieces of the code in sql_load.cc into virtual implementations of these Load_data_outvar derived classes.

Code blocks like this (they repeat multiple times):

      Item *real_item= item->real_item();

      if (item->type() == Item::STRING_ITEM)

      {

        ((Item_user_var_as_out_param *)item)->set_value((char*) pos, length,

                                                        read_info.read_charset);

      }

      else if (!real_item)

      {

        my_error(ER_NONUPDATEABLE_COLUMN, MYF(0), item->name);

        DBUG_RETURN(1);

      }

      else

      {

        ...

        field->store((char *) tag->value.ptr(), tag->value.length(), cs);

        ...

      }

will be simplified to one line:

  item->load_data_set_value(...)

Additionally, replacing dangerous tests for Item::STRING_ITEM and dangerous casts to Item_user_var_as_out_param to virtual calls will remove potential bugs like MDEV-12696, when the code works fine for LOAD DATA but crashes/fails for LOAD XML.