Protobuf serialization drops `preserve_partitioning` from `SortExec`

[thinkharderdev]

Describe the bug
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When SortExec is serialized to protobuf the preserve_partitioning flag is dropped and always set to false when deserializing.

To Reproduce
Steps to reproduce the behavior:

Do a round-trip serialization on a SortExec plan

Expected behavior
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The flag should be preserved in the serialized version

Additional context
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