Add PDF/A-3 output with embedded files for ZUGFeRD e-invoicing

Objo can create standard PDFs today through printing, report output, and chart export. PDF/A-3 is the archival PDF variant whose defining feature is that it permits embedded files, and it is the container format for e-invoicing standards such as ZUGFeRD and Factur-X, where an EN 16931 invoice XML travels inside the visual PDF invoice.

This feature adds PDF/A-3 output to Objo's own PDF generation, with support for embedding caller-supplied files such as invoice XML and attaching the metadata those standards require. Users generate their XML with whatever tooling they already trust and hand it to Objo to embed into a conformant PDF/A-3 document; constructing or validating the invoice XML itself is out of scope.

This enables small businesses to produce legally compliant e-invoices (mandatory for German B2B invoicing from 2027) entirely within Objo, without commercial third-party plugins.