Add async parameter to @Subscribe, also as a migration aid.

api/src/main/java/com/velocitypowered/api/event/Subscribe.java

@@ -26,4 +26,23 @@ public @interface Subscribe {
    */
   PostOrder order() default PostOrder.NORMAL;
 
+  /**
+   * Whether the handler must be called asynchronously.
+   *
+   * <p><strong>This option currently has no effect, but in the future it will. In Velocity 3.0.0,
+   * all event handlers run asynchronously by default. You are encouraged to determine whether or
+   * not to enable it now. This option is being provided as a migration aid.</strong></p>
+   *
+   * <p>If this method returns {@code true}, the method is guaranteed to be executed
+   * asynchronously. Otherwise, the handler may be executed on the current thread or
+   * asynchronously. <strong>This still means you must consider thread-safety in your
+   * event listeners</strong> as the "current thread" can and will be different each time.</p>
+   *
+   * <p>If any method handler targeting an event type is marked with {@code true}, then every
+   * handler targeting that event type will be executed asynchronously.</p>
+   *
+   * @return Requires async
+   */
+  boolean async() default true;
+
 }