[Enhancement] (nereids)implement showTriggersCommand in nereids

[LiBinfeng-01]

showTriggers command is supported in legacy planner but not in nereids(new doris fe planner). you need implement the following showTriggers command in nereids:
unsupportedshowTriggersStatement
SHOW FULL? TRIGGERS ((FROM | IN) database=multipartIdentifier)? wildWhere?

[Vallishp]

i will pick it