feat(pptx): partition_pptx() accepts strategy arg (#2879)
**Summary**
As we move to adding pluggable sub-partitioners, `partition_pptx()` will
need to become sensitive to the `strategy` argument, in particular when
it is set to "hi_res". Up until now there were no expensive operations
(inference, OCR, etc.) incurred while partitioning PPTX so this argument
was ignored.
After this PR, `partition_pptx()` still won't do anything with that
value, other than pass it along to `_PptxPartitionerOptions` for
safe-keeping, but now its ready for use by a `PicturePartitioner` (to
come in a subsequent PR).
