Added callback line of sight methods. Thanks xZise!

By: EvilSeph <evilseph@gmail.com>

paper-api/src/main/java/org/bukkit/util/Callback.java

@@ -0,0 +1,19 @@
+package org.bukkit.util;
+
+/**
+ * Primitive callback class, to allow specific callback handling. For example to determine if a block in sight is invisible.
+ *
+ * @param <Result> This is the type it will return.
+ * @param <Parameter> This is the type it has as parameter.
+ */
+public interface Callback<Result, Parameter> {
+
+    /**
+     * This method will be called on each step.
+     *
+     * @param parameter The parameter of this step.
+     * @return The result of the step.
+     */
+    public Result call(Parameter parameter);
+
+}