[Enhancement] (nereids)implement showEventsCommand in nereids

[LiBinfeng-01]

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

[Vallishp]

i will pick it