Add support for fetch overlay/underlay (#69)

Author: nitzanj

cloudinary-core/src/main/java/com/cloudinary/transformation/FetchLayer.java

@@ -0,0 +1,25 @@
+package com.cloudinary.transformation;
+
+import com.cloudinary.utils.Base64Coder;
+
+public class FetchLayer extends AbstractLayer<FetchLayer> {
+
+    public FetchLayer() {
+        this.type = "fetch";
+    }
+
+    public FetchLayer url(String remoteUrl) {
+        this.publicId = Base64Coder.encodeString(remoteUrl);
+        return this;
+    }
+
+    @Override
+    public FetchLayer type(String type) {
+        throw new UnsupportedOperationException("Cannot modify type for fetch layers");
+    }
+
+    @Override
+    FetchLayer getThis() {
+        return this;
+    }
+}

cloudinary-core/src/test/java/com/cloudinary/test/CloudinaryTest.java

@@ -988,7 +988,11 @@ public void testOverlayOptions() {
                 "text:Arial_18_bold_italic_letter_spacing_4_line_spacing_3:Hello%20World%252C%20Nice%20to%20meet%20you%3F",
                 new SubtitlesLayer().publicId("sample_sub_en.srt"), "subtitles:sample_sub_en.srt",
                 new SubtitlesLayer().publicId("sample_sub_he.srt").fontFamily("Arial").fontSize(40),
-                "subtitles:Arial_40:sample_sub_he.srt"};
+                "subtitles:Arial_40:sample_sub_he.srt",
+                new FetchLayer().url("https://test").resourceType("image"),
+                "fetch:aHR0cHM6Ly90ZXN0",
+                new FetchLayer().url("https://test"),
+                "fetch:aHR0cHM6Ly90ZXN0"};
 
         for (int i = 0; i < tests.length; i += 2) {
             Object layer = tests[i];