Add reachability metadata for RSocket client

Prior to this commit, using the RSocket client in a native image would
fail when setting up the codecs, if the chosen transport was WebSocket.
This is due to the fact that the `GraphQlWebSocketMessage` type was not
registered for reflection and the proper codec could not be detected as
a result.

This commit adds reflection metadata for this type.

Fixes gh-574

Author: bclozel

spring-graphql/src/main/resources/META-INF/native-image/org.springframework.graphql/spring-graphql/reflect-config.json

@@ -5,7 +5,16 @@
     "allDeclaredMethods":true,
     "allDeclaredConstructors":true,
     "condition": {
-      "typeReachable": "org.springframework.graphql.client.WebSocketGraphQlTransport"
+      "typeReachable": "org.springframework.graphql.client.DefaultRSocketGraphQlClientBuilder"
+    }
+  },
+  {
+    "name":"org.springframework.graphql.server.support.GraphQlWebSocketMessage",
+    "allDeclaredFields":true,
+    "allDeclaredMethods":true,
+    "allDeclaredConstructors":true,
+    "condition": {
+      "typeReachable": "org.springframework.graphql.client.DefaultWebSocketGraphQlClientBuilder"
     }
   }
 ]