feat: add retireval_top_n to config in env

[ProseGuys]

Summary

Solution: Add a new retrieval-related parameter top_n in the environment variables. During the retrieval process, use the default top_k parameter to obtain the top_k most relevant slices. When reranking, use the top_n parameter to return the top_n slices.

[!Tip] Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details. Resolves #11068

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Checklist

[!IMPORTANT]
Please review the checklist below before submitting your pull request.

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