feat(Extensions): Add PDF editing extension methods using iText

- Introduced `Edit` extension methods for `Stream` and `byte[]` to allow
  in-place PDF modifications via a `PdfDocument` action.
- Supports both .NET and .NET Framework projects.
- Ensures proper disposal of streams and PdfDocument instances.

EnvelopeGenerator.PdfEditor/Extensions.cs

@@ -0,0 +1,42 @@
+using iText.Kernel.Colors;
+using iText.Kernel.Pdf;
+using iText.Kernel.Pdf.Canvas;
+
+#if NETFRAMEWORK
+using System.IO;
+using System;
+#endif
+
+namespace EnvelopeGenerator.PdfEditor
+#if NET
+    ;
+#elif NETFRAMEWORK
+{
+#endif
+
+    public static class Extensions
+    {
+        public static TStream Edit<TStream>(this TStream inputStream, Action<PdfDocument> edit)
+            where TStream : Stream, new()
+        {
+            using (var outputStream = new TStream())
+            {
+                using (var pdfDoc = new PdfDocument(new PdfReader(inputStream), new PdfWriter(outputStream)))
+                {
+                    edit(pdfDoc);
+                }
+                return outputStream;
+            }
+        }
+
+        public static byte[] Edit(this byte[] pdfBytes, Action<PdfDocument> edit)
+        {
+            using (var inputStream = new MemoryStream(pdfBytes))
+            {
+                return inputStream.Edit(edit).ToArray();
+            }
+        }
+    }
+#if NETFRAMEWORK
+}
+#endif