SqlBulkCopy: Add hooks to enable retry strategies on batches

If SqlBulkCopy encounters any kind of exception as it inserts rows into a table, it fails the entire job.

In some scenarios, certain errors might be appropriate to retry - for instance, a deadlock or timeout. While this might be unlikely when doing bulk inserts to a staging table, it is much more likely if the table is also shared by a web application, for instance.

For this reason, until now we've instead used Table Valued Parameters.

It would be helpful though if SqlBulkCopy gave us a mechanism to apply a retry strategy as appropriate.

_If this isn't the appropriate place to post suggestions like this, let me know. If there was interest in the above, I would happily investigate the feasibility of this further._