Wrap rest httpclient with doPrivileged blocks (#22603)

This is related to #22116. A number of modules (reindex, etc) use the
rest client. The rest client opens connections using the apache http
client. To avoid throwing SecurityException when using the
SecurityManager these operations must be privileged. This is tricky
because connections are opened within the httpclient code on its
reactor thread. The way I confronted this was to wrap the creation
of the client (and creation of reactor thread) in a doPrivileged
block. The new thread inherits the existing security context.

client/rest/src/main/java/org/elasticsearch/client/RestClientBuilder.java

@@ -28,6 +28,8 @@
 import org.apache.http.impl.nio.client.HttpAsyncClientBuilder;
 import org.apache.http.nio.conn.SchemeIOSessionStrategy;
 
+import java.security.AccessController;
+import java.security.PrivilegedAction;
 import java.util.Objects;
 
 /**
@@ -177,7 +179,12 @@ public RestClient build() {
         if (failureListener == null) {
             failureListener = new RestClient.FailureListener();
         }
-        CloseableHttpAsyncClient httpClient = createHttpClient();
+        CloseableHttpAsyncClient httpClient = AccessController.doPrivileged(new PrivilegedAction<CloseableHttpAsyncClient>() {
+            @Override
+            public CloseableHttpAsyncClient run() {
+                return createHttpClient();
+            }
+        });
         RestClient restClient = new RestClient(httpClient, maxRetryTimeout, defaultHeaders, hosts, pathPrefix, failureListener);
         httpClient.start();
         return restClient;