Feat(frontend): add a column width session variable for explaination

[st1page]

We have introduce a class to generate pretty format of explain's result. with a column width, it will give a better format.
https://github.com/risingwavelabs/risingwave/blob/main/src/frontend/src/optimizer/plan_node/mod.rs#L670

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[xiangjinwu]

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[st1page]

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