Add ConfigurationSerializable-Serializable compatibility. Adds BUKKIT-4662

This commit adds a comaptibility layer for use between
ConfigurationSerializable and Java Serializable, such that when using the
Bukkit object streams, any ConfigurationSerializable acts as if it
implements Serializable for purposes of that wrapped stream.

Included are a set of unit tests for the stream with a check for backward
compatibility across versions.

src/main/javadoc/org/bukkit/util/io/package-info.java

@@ -0,0 +1,6 @@
+/**
+ * Classes used to facilitate stream processing for specific Bukkit concepts.
+ * <p>
+ */
+package org.bukkit.util.io;
+