Storage Transfer Service has updated transfer agent behavior when transferring to Cloud Storage

## Change Storage Transfer Service has updated transfer agent behavior when transferring to Cloud Storage. To align with Cloud Storage [auto-scaling](https://cloud.google.com/storage/docs/request-rate#auto-scaling), agents now gradually ramp up the number of requests being made to Cloud Storage. Customers who transfer many small files may notice initially slower transfer speeds while the requests are ramping up, but increased performance across the duration of the transfer.