feat(pdf): Support for password-protected PDF documents

[dolfim-ibm]

Allow to process password-protected PDF documents.

Still TODO:

• Pin new version of docling-parse
• Use the new backend_options introduced in feat(backend): add generic options support and HTML image handling modes #2011
• Add pdf-password option for CLI

Refactor of backend options:

• Extend backend_options to all document backends
• Simplify logic as "if you set backend_options, you have to also set a backend which supports them"

Issue resolved by this Pull Request:
Resolves #1081

Checklist:

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

[mergify]

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[github-actions]

✅ DCO Check Passed

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[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/cli/main.py	60.00%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[PeterStaar-IBM]

lgtm!

[dosubot]

Documentation Updates

Checked 3 published document(s). No updates required.

^{How did I do? Any feedback?}  

[ceberam]

The overall backend options look now very robust.