[Enhancement] (nereids)implement showIndexCommand in nereids

[LiBinfeng-01]

showIndex command is supported in legacy planner but not in nereids(new doris fe planner). you need implement the following showIndex command in nereids:
unsupportedshowIndexStatement

        (FROM |IN) tableName=multipartIdentifier
        ((FROM | IN) database=multipartIdentifier)?                                 ```

[Dragonliu2018]

i want to try it.

[github-actions]

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