Make VertxAsyncExec public

This is mainly to fix javadoc error: `No public or protected classes found to document`, but it also aligns with `JavaPoolAsyncExec` being public.

Author: dimas-b

persistence/nosql/async/vertx/src/main/java/org/apache/polaris/nosql/async/vertx/VertxAsyncExec.java

@@ -56,7 +56,7 @@
  * a Java thread pool to execute blocking tasks.
  */
 @ApplicationScoped
-class VertxAsyncExec implements AsyncExec, AutoCloseable {
+public class VertxAsyncExec implements AsyncExec, AutoCloseable {
   private static final Logger LOGGER = LoggerFactory.getLogger(VertxAsyncExec.class.getName());
   private static final Duration MAX_DURATION = Duration.ofDays(7);