Use time-constant comparison for CSRF tokens in endpoint (#10157)

This hardens the framework against a theoretical timing attack based on
comparing how quickly a request with an invalid CSRF token is rejected.

No tests since this functionality is equivalent to the previous implementation aside from timing differences that would be very fragile to verify in an automated test.

Related to #9875

flow-server/src/main/java/com/vaadin/flow/server/connect/auth/VaadinConnectAccessChecker.java

@@ -24,6 +24,8 @@
 import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -158,7 +160,10 @@ private boolean requestForbidden(HttpServletRequest request) {
             return true;
         }
 
-        if (!csrfTokenInSession.equals(request.getHeader("X-CSRF-Token"))) {
+        String csrfTokenInRequest = request.getHeader("X-CSRF-Token");
+        if (csrfTokenInRequest == null || !MessageDigest.isEqual(
+                csrfTokenInSession.getBytes(StandardCharsets.UTF_8), 
+                csrfTokenInRequest.getBytes(StandardCharsets.UTF_8))) {
             if (getLogger().isInfoEnabled()) {
                 getLogger().info("Invalid CSRF token in endpoint request");
             }