Expose some functions to support PAX for partition tables

[gfphoenix78]

1. ComputePartitionAttrs: expose function
2. transformPartitionBound: add argument of PartitionKey
3. transformPartitionRangeBounds: add argument of PartitionKey
4. list_sort_arg: sort the List with context argument in cmp function

No behavior changes, no test.

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[my-ship-it]

LGTM