fix: expose Spring executor for Vaadin dev mode (#692)

Part of #691

# Conflicts:
#	vaadin-spring/src/test/java/com/vaadin/flow/spring/VaadinServletContextInitializerTest.java

vaadin-spring/src/main/java/com/vaadin/flow/spring/VaadinServletContextInitializer.java

@@ -36,6 +36,7 @@
 import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
+import java.util.concurrent.Executor;
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
@@ -56,6 +57,7 @@
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.ResourceLoader;
 import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
+import org.springframework.core.task.TaskExecutor;
 import org.springframework.core.type.filter.AnnotationTypeFilter;
 import org.springframework.core.type.filter.AssignableTypeFilter;
 
@@ -343,6 +345,8 @@ public void contextInitialized(ServletContextEvent event) {
                     || !config.enableDevServer()) {
                 return;
             }
+            config.getInitParameters().put(Executor.class,
+                    appContext.getBean(TaskExecutor.class));
 
             Set<String> basePackages;
             if (isScanOnlySet()) {