Implemented core logic

GifResizer/Service/ResizeService.cs

@@ -1,6 +1,20 @@
-namespace GifResizer.Service;
+using GifResizer.Models;
+
+namespace GifResizer.Service;
 
 public class ResizeService
 {
-    
+
+    public ResizeService()
+    {
+        
+    }
+
+    public void ResizeGif(string filePath, int width, int height)
+    {
+        using Gif gif = new Gif(filePath);
+        gif.Load();
+        gif.Resize(width, height);
+        gif.Save("D:/test-output.gif");
+    }
 }