Re-add hudson.UDPBroadcastFragment

The class was removed with f10d9f9 ("[JENKINS-60913] Remove network
discovery services (jenkinsci#4460)") but is required by the swarm client. As
result, swarm clients are unable use Jenkins:

java.lang.NoClassDefFoundError: hudson/UDPBroadcastFragment
        at hudson.plugins.swarm.PluginImpl.getSwarmSecret(PluginImpl.java:262)

This commit re-adds hudson.UDPBroadcastFragment to jenkin's core, to
allow swarm client to the use the master again and hence fixes
JENKINS-61029.

core/src/main/java/hudson/UDPBroadcastFragment.java

@@ -0,0 +1,58 @@
+/*
+ * The MIT License
+ *
+ * Copyright (c) 2004-2009, Sun Microsystems, Inc., Kohsuke Kawaguchi
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package hudson;
+
+
+import java.net.SocketAddress;
+
+/**
+ * Extension point that contributes an XML fragment to the UDP broadcast.
+ *
+ * <p>
+ * Put {@link Extension} on your implementation class to have it auto-discovered.
+ *
+ * @author Kohsuke Kawaguchi
+ * @since 1.304
+ * @see UDPBroadcastThread
+ */
+public abstract class UDPBroadcastFragment implements ExtensionPoint {
+    /**
+     * Called to build up a response XML.
+     *
+     * @param buf
+     *      This is the buffer to write XML to. The implementation of this method
+     *      should write a complete fragment. Because of the packet length restriction
+     *      in UDP (somewhere around 1500 bytes), you cannot send a large amount of information.
+     * @param sender
+     *      The socket address that sent the discovery packet out.
+     */
+    public abstract void buildFragment(StringBuilder buf, SocketAddress sender);
+
+    /**
+     * Returns all the registered {@link UDPBroadcastFragment}s.
+     */
+    public static ExtensionList<UDPBroadcastFragment> all() {
+        return ExtensionList.lookup(UDPBroadcastFragment.class);
+    }
+}