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/gotenberg/pdfengine.go

@@ -88,6 +88,8 @@ type PdfFormats struct {
 // PdfEngine provides an interface for operations on PDFs. Implementations
 // can utilize various tools like PDFtk, or implement functionality directly in
 // Go.
+//
+//nolint:dupl
 type PdfEngine interface {
 	// Merge combines multiple PDFs into a single PDF. The resulting page order
 	// is determined by the order of files provided in inputPaths.
@@ -96,6 +98,11 @@ type PdfEngine interface {
 	// Split splits a given PDF file.
 	Split(ctx context.Context, logger *zap.Logger, mode SplitMode, inputPath, outputDirPath string) ([]string, error)
 
+	// Flatten merges existing annotation appearances with page content, effectively deleting the original annotations.
+	// This process can flatten forms as well, as forms share a relationship with annotations.
+	// Note that this operation is irreversible.
+	Flatten(ctx context.Context, logger *zap.Logger, inputPath string) error
+
 	// Convert transforms a given PDF to the specified formats defined in
 	// PdfFormats. If no format, it does nothing.
 	Convert(ctx context.Context, logger *zap.Logger, formats PdfFormats, inputPath, outputPath string) error