feat: add simple pptx_extractor for etl_type=dify

[kurokobo]

Summary

This PR introduces simple PPTXExtractor to support PPTX files in the Knowledge in the environment with ETL_TYPE=dify (which is default deployment).

By referring existing WordExtractor with python-docx, this PR implements simple text-extaction method for PPTX:

• Using python-pptx by the same maintainer of python-docx.
• Based on the WordExtractor with minimal modification.
• Support extracting images, tables, and hyperlinks (with http or https) in the slides.

Closes #24182

Screenshots

Example PPTX file (includes some images and tables): https://create.microsoft.com/en-us/template/business-plan-presentation-96423eb0-8e02-4a99-b83b-bc60d19202d5

Before	After
	PPTX is accepted: Preview of Chunks, works fine for images and tables:

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[kurokobo]

I think this could serve as a temporary solution, at least until a RAG 2.0 is implemented and becomes widely adopted in the community—even if it gets merged.
Even if it doesn't get merged, I believe there is value in making my code public for users who want to customize things themselves 😃

[kurokobo]

VDB tests are failing, but it seems to be unrelated to my PR. Maybe it will work if we just rerun it?

[kurokobo]

Re-opened to trigger CI again🙏

[kurokobo]

Hmm, these tests are a bit flaky. Both tests fail for unrelated reasons, such as docker pull and internal HTTP requests.

Let me know if there is something I should do, thanks!

[kurokobo]

Wow, all checks have passed, thanks @crazywoola for re-running.

[crazywoola]

Hey @kurokobo I merged this extractor in the official-plugin-repo, talked to our team, since rag 2.0 will be released by the end of the month, so we can close it and wait for the new version.