Add getMetadata to the AdminClient

It seems like `getMetadata` should be available on the AdminClient. It would simplify managing topics from one client instance. The only other option is to spin up a producer or consumer client and use it's getMetadata method ( or the `connect` callback ) to see what topics exist , which seems to be a bit of a hack. I know the Go client has an AdminClient.getMetadata method. It would be nice to have it here too.