Return Vec<bool> instead of bool in ExecutionPlan::maintains_input_order

[mustafasrepo]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Currently API of maintains_input_order is as follows fn maintains_input_order(&self) -> bool. It returns a boolean indicating whether the executor keeps ordering. Although this is sufficient for most of the executors. For executors with multiple children, this API is insufficient. Consider UnionExec with two children where ordering of children are Sort(a, b) and Sort(a) respectively. The output will be sorted with Sort(a). However, in current API we return false(Returning true would be wrong). However, if we were to update API to return Vec<bool>. We could return vec![false, true]. This will enable us to further optimize physical plans.

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