MDEV-32879 Server crash in my_decimal::operator= or unexpected ER_DUP_ENTRY upon comparison with INET6 and similar types

During the 10.5->10.6 merge please use the 10.6 code on conflicts.

This is the 10.5 version of the patch (a backport of the 10.6 version).
Unlike 10.6 version, it makes changes in plugin/type_inet/sql_type_inet.*
rather than in sql/sql_type_fixedbin.h

Item_bool_rowready_func2, Item_func_between, Item_func_in
did not check if a not-NULL argument of an arbitrary data type
can produce a NULL value on conversion to INET6.

This caused a crash on DBUG_ASSERT() in conversion failures,
because the function returned SQL NULL for something that
has Item::maybe_null() equal to false.

Adding setting NULL-ability in such cases.

Details:

- Removing the code in Item_func::setup_args_and_comparator()
  performing character set aggregation with optional narrowing.
  This aggregation is done inside Arg_comparator::set_cmp_func_string().
  So this code was redundant

- Removing Item_func::setup_args_and_comparator() as it git simplified to
  just to two lines:
    convert_const_compared_to_int_field(thd);
    return cmp->set_cmp_func(thd, this, &args[0], &args[1], true);
  Using these lines directly in:
    - Item_bool_rowready_func2::fix_length_and_dec()
    - Item_func_nullif::fix_length_and_dec()

- Adding a new virtual method:
  - Type_handler::Item_bool_rowready_func2_fix_length_and_dec().

- Adding tests detecting if the data type conversion can return SQL NULL into
  the following methods of Type_handler_inet6:
  - Item_bool_rowready_func2_fix_length_and_dec
  - Item_func_between_fix_length_and_dec
  - Item_func_in_fix_comparator_compatible_types

sql/sql_type.h

@@ -55,6 +55,7 @@ class Item_hybrid_func;
 class Item_func_min_max;
 class Item_func_hybrid_field_type;
 class Item_bool_func2;
+class Item_bool_rowready_func2;
 class Item_func_between;
 class Item_func_in;
 class Item_func_round;
@@ -3233,9 +3234,15 @@ public:
   bool agg_item_collations(DTCollation &c, const char *name,
                            Item **items, uint nitems,
                            uint flags, int item_sep);
+  struct Single_coll_err
+  {
+    const DTCollation& coll;
+    bool first;
+  };
   bool agg_item_set_converter(const DTCollation &coll, const char *fname,
                               Item **args, uint nargs,
-                              uint flags, int item_sep);
+                              uint flags, int item_sep,
+                              const Single_coll_err *single_item_err= NULL);
 
   /*
     Collect arguments' character sets together.
@@ -4232,6 +4239,8 @@ public:
   }
   virtual bool Item_eq_value(THD *thd, const Type_cmp_attributes *attr,
                              Item *a, Item *b) const= 0;
+  virtual bool Item_bool_rowready_func2_fix_length_and_dec(THD *thd,
+                                          Item_bool_rowready_func2 *func) const;
   virtual bool Item_hybrid_func_fix_attributes(THD *thd,
                                                const char *name,
                                                Type_handler_hybrid_field_type *,