Add optional polling interval/timeout options to step PullFromSFTP

There are instances when we need a workflow to poll for files on an SFTP site. We use the PullFromSFTP step to do this, but currently we're having to use loops and counters in workflows to accomodate the polling. Having this built into the step would be much more efficient, simple and less confusing. Thinking the polling options would be very similar to those in the RunHotfolderApplication step, but would have an additional option to turn off polling altogether if not needed.