feat: Extracting picture data for raster images found in PPTX

[maxmnemonic]

This PR populates image data in docling documents by PPTX backend, also introduces basic PPTX tests.

Checklist:

• [ ] Documentation has been updated, if necessary.
• [ ] Examples have been added, if necessary.
• [x] Tests have been added, if necessary.

[mergify[bot]]

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🟢 Require two reviewer for test updates

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[maxmnemonic]

Added tests, ready for re-review