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/routes.go

@@ -60,6 +60,7 @@ func convertRoute(libreOffice libreofficeapi.Uno, engine gotenberg.PdfEngine) ap
 				maxImageResolution              int
 				nativePdfFormats                bool
 				merge                           bool
+				flatten                         bool
 			)
 
 			err := form.
@@ -135,6 +136,7 @@ func convertRoute(libreOffice libreofficeapi.Uno, engine gotenberg.PdfEngine) ap
 					}
 					return nil
 				}).
+				Bool("flatten", &flatten, false).
 				Validate()
 			if err != nil {
 				return fmt.Errorf("validate form data: %w", err)
@@ -261,6 +263,13 @@ func convertRoute(libreOffice libreofficeapi.Uno, engine gotenberg.PdfEngine) ap
 				return fmt.Errorf("write metadata: %w", err)
 			}
 
+			if flatten {
+				err = pdfengines.FlattenStub(ctx, engine, outputPaths)
+				if err != nil {
+					return fmt.Errorf("flatten PDFs: %w", err)
+				}
+			}
+
 			if len(outputPaths) > 1 && splitMode == zeroValuedSplitMode {
 				// If .zip archive, document.docx -> document.docx.pdf.
 				for i, inputPath := range inputPaths {