[Feature] Fold constant expr by Backend

[qidaye]

Now when Doris is performing some constant expr, if the data query is not involved, it is calculated directly in FE,

eg: select str_to_date(202009,'%Y%m'); calculate in FE
select str_to_date(str,'%Y%m') from tb; calculate in BE

Now two sets of functions are maintained, and it is easy to cause inconsistent FE/BE calculation results.

Plan to push down the FE calculation to BE. Keep the calculation results consistent to facilitate code maintenance.