feat(pdf): Support for password-protected PDF documents (#2499)

* add test and example for PDF with password

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* use docling-parse with new password feature

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add pdfbackendoptions

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* generalize backend_options and add PdfBackendOptions

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* add pdf-password option

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* update exception test

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

* fix docs description

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

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Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

tests/data/pdf_password/README.md

@@ -0,0 +1,4 @@
+This folder contains test documents which are locked.
+
+- Opening password: `1234`
+- Owner password: `owner`