Add long text type STRING, with a maximum length of 2GB. Usage is similar to varchar, and there is no guarantee for the performance of storing extremely long data (#6391)

be/src/olap/key_coder.cpp

@@ -71,6 +71,7 @@ private:
         add_mapping<OLAP_FIELD_TYPE_DECIMAL>();
         add_mapping<OLAP_FIELD_TYPE_CHAR>();
         add_mapping<OLAP_FIELD_TYPE_VARCHAR>();
+        add_mapping<OLAP_FIELD_TYPE_STRING>();
         add_mapping<OLAP_FIELD_TYPE_BOOL>();
     }