Migrate gotify to new client

app/src/main/java/com/github/gotify/api/Api.java

@@ -1,4 +1,21 @@
 package com.github.gotify.api;
 
+import java.io.IOException;
+import retrofit2.Call;
+import retrofit2.Response;
+
 public class Api {
+    public static <T> T execute(Call<T> call) throws ApiException {
+        try {
+            Response<T> response = call.execute();
+
+            if (response.isSuccessful()) {
+                return response.body();
+            } else {
+                throw new ApiException(response);
+            }
+        } catch (IOException e) {
+            throw new ApiException(e);
+        }
+    }
 }