Replace logical plan from Arc<T> to Box<T>

[jayzhan211]

Which issue does this PR close?

Closes #.

Rationale for this change

#9637 I think it is easier to avoid clone after the plan are all boxed instead of arced

What changes are included in this PR?

1. Change Arc to Box
2. union is changed to Vec<T> instead of Vec<Box<T>>

Are these changes tested?

Are there any user-facing changes?

[jayzhan211]

It is part of #9768 , if #9768 is too complex to review, we can merge this first.

[alamb]

Thanks @jayzhan211 -- I only just saw that you were working on this -- I agree this would be a good change. However, I think it will be a substantial performance regression until we also avoid so many logical plan clone's in the optimizer

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