Implement pre-spawn API to allow modifications to spawned entities.

See preceding commit for why this change was included.

By: Xor Boole <mcyoung@mit.edu>

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

@@ -0,0 +1,17 @@
+package org.bukkit.util;
+
+/**
+ * Represents an operation that accepts a single input argument and returns no
+ * result.
+ *
+ * @param <T> the type of the input to the operation
+ */
+public interface Consumer<T> {
+
+    /**
+     * Performs this operation on the given argument.
+     *
+     * @param t the input argument
+     */
+    void accept(T t);
+}