Give transport-nio plugin socket permissions (#28900)

This is related to #27260. The transport-nio plugin needs socket
permissions to operate as a transport. This commit gives it these
permissions in the policy file.

plugins/transport-nio/src/main/plugin-metadata/plugin-security.policy

@@ -0,0 +1,23 @@
+/*
+ * 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.
+ */
+
+grant codeBase "${codebase.elasticsearch-nio}" {
+  // elasticsearch-nio makes and accepts socket connections
+  permission java.net.SocketPermission "*", "accept,connect";
+};