This commit will add a new sql mode named MODE_PIPES_AS_CONCAT:
Description:
1、If this mode is active, '||' will be handled different from the original way ('||' and 'or' are seen as the same symbols in Doris) that it can be used to concat two exps and returns a new string. For example, 'a' || 'b' = 'ab' and 1 || 0 = '10'.
2. User can active this mode by "SET sql_mode = PIPES_AS_CONCAT", and deactive it by "SET sql_mode = '' ".
At present, we do not support SQL MODE which is similar to MySQL. In MySQL, SQL MODE is stored in global session and session with a 64 bit address，and every bit 0 or 1 on this address represents a mode state. Besides, MySQL supports combine mode which is composed of several modes.
We should support SQL MODE to deal with sql dialect problems. We can heuristically use the MySQL way to store SQL MODE in session and parse it into string when we need to return it back to client.
This commit suggests a solution to support SQL MODE. But it's just a sample, and the mode types in SqlModeHelper.java are not really meaningful from now on.
