implement PdfFileAttachmentAnnotation

[julienrffr]

Implements PDF 1.3's FileAttachment annotation according to specs.
Will display a clickable annotation that opens an attachment, displaying either as an predefined icon or as a custom appearance.

Usage is simple (2 possibilities):
`var path = @"C:\Users\userxxx\Downloads\avion2.jpg";

        var fileSpec = new PdfFileSpecification(document, new PdfEmbeddedFileStream(document, File.OpenRead(path)), "avion111.jpg");
        var rectangleBottomBased = gfx.Transformer.WorldToDefaultPage(new XRect(new XPoint(30, 30), new XSize(30, 30)));
        var annot1 = page.AddFileAttachmentAnnotation(new PdfRectangle(rectangleBottomBased), fileSpec);

        var rectangleBottomBased2 = gfx.Transformer.WorldToDefaultPage(new XRect(new XPoint(300, 160), new XSize(250, 250)));
        var annot2 = page.AddFileAttachmentAnnotation(new PdfRectangle(rectangleBottomBased2), new PdfEmbeddedFileStream(document, File.OpenRead(path)), "avion222.jpg");
        annot2.CustomAppearanceHandler = new SignAppearenceHandler();
        annot2.RenderCustomAppearance();`

Either add an annotation from a known PdfFileSpecification (you can re-use it in other annotations), or directly from a PdfEmbeddedFileStream (PdfFileSpecification will be silently created).

HelloWorld-3BA27568D294498F9DB86F2E19A94AA2_tempfile.pdf

fileattachmentannotation.cs.txt

[julienrffr]

NB: this feature is available in PDFsharp-extended nuget package