Added PolyCustom feature for server functions.

Author: gastonortiz

library/src/main/java/io/polyapi/client/api/model/function/PolyCustom.java

@@ -0,0 +1,13 @@
+package io.polyapi.client.api.model.function;
+
+import io.polyapi.commons.api.model.PolyObject;
+import lombok.Getter;
+import lombok.Setter;
+
+/**
+ * Utility class that provides additional functionality to server functions. Available only during execution time.
+ */
+@Getter
+public class PolyCustom implements PolyObject {
+    private String environmentId;
+}