Failure to wait for events to be sent when client closes ... GcpLineageTransportConfig

I have a Java application that is loosely as follows:

```
GcpLineageTransportConfig gcpLineageTransportConfig = new GcpLineageTransportConfig();
gcpLineageTransportConfig.setProjectId("**********");
gcpLineageTransportConfig.setLocation("us-central1");

OpenLineageClient client = OpenLineageClient.
builder().
transport(new GcpLineageTransport(gcpLineageTransportConfig)).
build();

...

client.emit(runStateUpdate);
client.close();
```

What I find is that the application does *not* send the event. If I change the logic to:

```
client.emit(runStateUpdate);
System.out.println("Sleeping 5 seconds");
Thread.sleep(1000*5);
client.close();
```

the event is sent. What I believe is happening is that since the default is to transmit the event asynchronously (I.e. immediate return from `client.emit(...)`), I believe that `client.close()` is not waiting for queued event emission requests to be sent/flushed/processed. I believe this to be an error. The call to `client.close()` should block/wait for queued emitted events to be processed before returning.