pushdown tidb truncate to tiflash

[guo-shaoge]

Enhancement

For now tiflash truncate use ClickHouse code, the first argument cannot be Decimal(check

tiflash/dbms/src/Flash/Coprocessor/DAGUtils.cpp

Line 314 in 1697c72

//{tipb::ScalarFuncSig::TruncateDecimal, "cast"},

), and the second argument can only be constant(check

tiflash/dbms/src/Functions/FunctionsRound.h

Line 686 in 1697c72

if (!scale_column.isColumnConst())

). (which is ok for floor/ceil because they only support one argument, but not ok for truncate, because it support two arguments).