feat(pdfengines): add support for flattening annotations (#1105)

* initial changes

* Add tests

* Fix edge case when we need to regenerate appearances

* Fix comments

* Add missing comment

* Add missing comment

* Add missing comment

* Add flatten option to the merge route

* Add flatten option to the libreoffice convert route

* Add flatten option to the chromium convert route

* Revert "Add flatten option to the chromium convert route"

This reverts commit cdab8b4e6b.

* Ignore lint false positives

* Add missing tests

* Add flatten route tests

* Replace input instead of creating a new file

* create copy before flatten in tests

---------

Co-authored-by: Peter Chakalov <peter.chakalov@abraxa.com>

pkg/modules/libreoffice/pdfengine/pdfengine.go

@@ -56,6 +56,11 @@ func (engine *LibreOfficePdfEngine) Split(ctx context.Context, logger *zap.Logge
 	return nil, fmt.Errorf("split PDF with LibreOffice: %w", gotenberg.ErrPdfEngineMethodNotSupported)
 }
 
+// Flatten is not available in this implementation.
+func (engine *LibreOfficePdfEngine) Flatten(ctx context.Context, logger *zap.Logger, inputPath string) error {
+	return fmt.Errorf("Flatten PDF with LibreOffice: %w", gotenberg.ErrPdfEngineMethodNotSupported)
+}
+
 // Convert converts the given PDF to a specific PDF format. Currently, only the
 // PDF/A-1b, PDF/A-2b, PDF/A-3b and PDF/UA formats are available. If another
 // PDF format is requested, it returns a [gotenberg.ErrPdfFormatNotSupported]