Use fetch limit in get_sorted_iter

[Dandandan]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
As @alamb suggested #3530 (comment) we can provide the fetch limit as well, reducing some of the work in the sort operator and the following SortPreservingMergeExec.

Describe the solution you'd like
Wire up fetch to lexsort_to_indices

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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