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>