Plugins: Add plugin extension capabilities (#27881)

This commit adds the infrastructure to plugin building and loading to
allow one plugin to extend another. That is, one plugin may extend
another by the "parent" plugin allowing itself to be extended through
java SPI. When all plugins extending a plugin are finished loading, the
"parent" plugin has a callback (through the ExtensiblePlugin interface)
allowing it to reload SPI.

This commit also adds an example plugin which uses as-yet implemented
extensibility (adding to the painless whitelist).

libs/plugin-classloader/src/main/java/org/elasticsearch/plugins/ExtendedPluginsClassLoader.java

@@ -0,0 +1,59 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.plugins;
+
+import java.security.AccessController;
+import java.security.PrivilegedAction;
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * A classloader that is a union over the parent core classloader and classloaders of extended plugins.
+ */
+public class ExtendedPluginsClassLoader extends ClassLoader {
+
+    /** Loaders of plugins extended by a plugin. */
+    private final List<ClassLoader> extendedLoaders;
+
+    private ExtendedPluginsClassLoader(ClassLoader parent, List<ClassLoader> extendedLoaders) {
+        super(parent);
+        this.extendedLoaders = Collections.unmodifiableList(extendedLoaders);
+    }
+
+    @Override
+    protected Class<?> findClass(String name) throws ClassNotFoundException {
+        for (ClassLoader loader : extendedLoaders) {
+            try {
+                return loader.loadClass(name);
+            } catch (ClassNotFoundException e) {
+                // continue
+            }
+        }
+        throw new ClassNotFoundException(name);
+    }
+
+    /**
+     * Return a new classloader across the parent and extended loaders.
+     */
+    public static ExtendedPluginsClassLoader create(ClassLoader parent, List<ClassLoader> extendedLoaders) {
+        return AccessController.doPrivileged((PrivilegedAction<ExtendedPluginsClassLoader>)
+            () -> new ExtendedPluginsClassLoader(parent, extendedLoaders));
+    }
+}