Add virtual Field::sp_prepare_and_store_item(), make sp_rcontext symmetric for scalar and ROW

After MDEV-14212, the Virtual_tmp_table instance that stores a ROW variable elements is accessible from the underlying Field_row (rather than Item_field_row).

Under terms of this task we'll do some further changes by moving the code from sp_instr_xxx, sp_rcontext, Item_xxx to Virtual_tmp_table and Field_xxx:

• Move the data type specific code (scalar vs ROW) which stores a value to an SP variable into a new virtual method Field_xxx::sp_prepare_and_store_item().
• Make the the code in sp_rcontext::set_variable() and sp_eval_expr() symmetric for scalar and ROW values.
• Make the code in in sp_rcontext::set_variable_row_field(), sp_rcontext::set_variable_row_field(), sp_rcontext::set_variable_row() symmetric for ROW elements (i.e. scalar and ROW elements inside a ROW).

Rationale:

Prepare the code to implement these tasks soon easier:

• MDEV-12252 ROW data type for stored function return values
• MDEV-12307 ROW data type for built-in function return values
• MDEV-6121 Data type: Array
• MDEV-10593 sql_mode=ORACLE: TYPE .. AS OBJECT: basic functionality
• ROW with ROW fields (no MDEV yet)