Allow SP variables as LOAD DATA out parameters

We currently allow user variables and SP variables as out parameters in:

• FETCH INTO @uservar vs FETCH INTO spvar
• CALL p1(@uservar_as_out_param) vs CALL p1(spvar_as_out_param)

But LOAD DATA is an exception:

It's possible to do:

LOAD DATA INFILE 'file.txt' INTO TABLE t1 (column1, @var1) SET column2 = @var1/100;

But its not possible to do inside a stored routine:

LOAD DATA INFILE 'file.txt' INTO TABLE t1 (column1, spvar1) SET column2 = spvar1/100;

It can be useful for stricter data type control in the SET clause.

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

• Derive Item_splocal, Item_splocal_row_field, Item_user_var_as_out_param, and possibly Item_trigger_field from Load_data_outvar (added in MDEV-15597).
• Possibly get rid of Item_user_var_as_out_param and replace it to Item_func_get_user_var or Item_func_get_user_var. This change is optional, we'll decide during development.
• Possibly change collecting LOAD DATA targets from List<Item> to List<Load_data_outvar>. So in case if we don't remove Item_user_var_as_out_param, we can at least remove its inheritance from Item and its useless methods like val_str, which currently do DBUG_ASSERT.