[Enhancement] (nereids)implement showCollationCommand in nereids

[LiBinfeng-01]

showCollation command is supported in legacy planner but not in nereids(new doris fe planner). you need implement the following showCollation command in nereids:
unsupportedshowCollationStatement
SHOW COLLATION wildWhere?

[msridhar78]

@starocean999 @LiBinfeng-01 Please assign this issue to me