GH-466: Export namespace from Flight package (#467)

Fixes #466.

Author: lidavidm

flight/flight-core/src/main/java/module-info.java

@@ -20,6 +20,7 @@
   exports org.apache.arrow.flight.auth;
   exports org.apache.arrow.flight.auth2;
   exports org.apache.arrow.flight.client;
+  exports org.apache.arrow.flight.grpc;
   exports org.apache.arrow.flight.impl;
   exports org.apache.arrow.flight.sql.impl;