MDEV-21263: Allow packed values of non-sorted fields in the sort buffer

This task deals with packing the non-sorted fields (or addon fields).
This would lead to efficient usage of the memory allocated for the sort buffer.
The changes brought by this feature are
  1) Sort buffers would have records of variable length
  2) Each record in the sort buffer would be stored like
     <sort_key1><sort_key2>....<addon_length><null_bytes><field1><field2>....
     addon_length is the extra bytes that are required to store the variable
     length of addon field across different records.
  3) Changes in rr_unpack_from_buffer and rr_from_tempfile to take into account
     the variable length of records.

Ported  WL#1509 Pack values of non-sorted fields in the sort buffer from
MySQL by Tor Didriksen

sql/sql_array.h

@@ -85,6 +85,10 @@ public:
 
   Element_type *array() const { return m_array; }
 
+  Element_type *begin() const { return array(); }
+  Element_type *end() const { return array() + m_size; }
+
+
   bool operator==(const Bounds_checked_array<Element_type>&rhs) const
   {
     return m_array == rhs.m_array && m_size == rhs.m_size;