Support CSRF protection in GraphiQL with cookie-to-header strategy

Prior to this commit, secured applications with CSRF protection could
not easily use the GraphiQL integration shipped with Spring for GraphQL,
as the JavaScript code would not work with any CSRF protection strategy
for single page apps.

This commit checks whether the main HTTP response contains a
`XSRF-TOKEN` Cookie value, and uses it as a `X-XSRF-TOKEN` request
header for the next AJAX request to the `/graphql` endpoint.

Note that a specific configuration must be set in Spring Security to
achieve that:
* the CSRF token must be sent as a response Cookie for the initial
  authenticated request
* if the application is protected against BREACH, all new token values
  must be sent as response cookies as well and a request handler must be
  configured

Closes gh-758

spring-graphql/src/main/resources/graphiql/index.html

@@ -30,19 +30,17 @@
     const wsPath = params.get("wsPath") || "/graphql";
     const wsProtocol = location.protocol === 'https:' ? 'wss:' : 'ws:';
     const subscriptionUrl = `${wsProtocol}//${location.host}${wsPath}`;
-
-    const gqlFetcher = GraphiQL.createFetcher({
-        url,
-        subscriptionUrl,
-    });
-
+    const gqlFetcher = GraphiQL.createFetcher({'url': url, 'subscriptionUrl': subscriptionUrl});
     const explorerPlugin = GraphiQLPluginExplorer.explorerPlugin();
+    const xsrfToken = document.cookie.match(new RegExp('(?:^| )XSRF-TOKEN=([^;]+)'));
+    const headers = xsrfToken ? `{ "X-XSRF-TOKEN" : "${ xsrfToken[1] }" }` : `{}`;
     ReactDOM.render(
         React.createElement(GraphiQL, {
             fetcher: gqlFetcher,
             defaultVariableEditorOpen: true,
             headerEditorEnabled: true,
             shouldPersistHeaders: true,
+            headers: headers,
             plugins: [explorerPlugin]
         }),
         document.getElementById('graphiql'),