Make FlinkKinesisFirehoseProducer#createKinesisFirehoseClient protected to allow for instrumentation

[bobtiernay-okta]

Currently FlinkKinesisFirehoseProducer#createKinesisFirehoseClient is package-private. This means it's not nice to create a refinement to instrument metrics, etc. against client operations. It would be great if the following were possible:

    /**
     * {@inheritDoc}
     * <p>See <a href="https://github.com/aws/aws-kinesisanalytics-flink-connectors/issues/15">aws-kinesisanalytics-flink-connector #15</a>
     * for description of why this is needed.
     */
    @Nonnull
    @Override
    protected AmazonKinesisFirehose createKinesisFirehoseClient() {
        return new MeteredAmazonKinesisFirehose(getRuntimeContext().getMetricGroup(), super.createKinesisFirehoseClient());
    }