Register sun.nio.ch.SelectorImpl fields for unsafe access. (#9631)

Motivation:

On JDK > 9 Netty uses Unsafe to write two internal JDK fields: sun.nio.ch.SelectorImp.selectedKeys and sun.nio.ch.SelectorImpl.publicSelectedKeys. This is done in transport/src/main/java/io/netty/channel/nio/NioEventLoop.java:225, in openSelector() method. The GraalVM analysis cannot do the Unsafe registration automatically because the object field offset computation is hidden behind two layers of calls.

Modifications:

This PR updates the Netty GraalVM configuration by registering those fields for unsafe access.
 
Result:

Improved support for Netty on GraalVM with JDK > 9.

transport/src/main/resources/META-INF/native-image/io.netty/transport/reflection-config.json

@@ -4,5 +4,12 @@
       "methods": [
         { "name": "<init>", "parameterTypes": [] }
       ]
+    },
+    {
+      "name": "sun.nio.ch.SelectorImpl",
+      "fields": [
+        { "name": "selectedKeys",  "allowUnsafeAccess" : true},
+        { "name": "publicSelectedKeys",  "allowUnsafeAccess" : true}
+      ]
     }
 ]