Avoid per-row allocations in to_char #20634
Description
Is your feature request related to a problem or challenge?
In both the scalar and array arg paths, to_char allocates a string for every row, and then does a final copy via StringArray::from to convert the results to Arrow format.
Instead, we can use a reusable buffer for the output of ArrayFormatter, and then append into a StringBuilder. That avoids the per-row allocation.
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Additional context
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