Add byte threshold batching settings to publisher

The publisher will need to be able to:
1. Calculate the size of a message
2. Check if a batch exceeds the limits
3. Ensure that the MAX limit is not exceeded by a batch (a message cannot be added to the batch if it would make it larger than the quota limits).
4. 
See https://cloud.google.com/pubsub/docs/batch-messaging#quotas_and_limits_on_batch_messaging