Add Snapshot TransactionIsolationLevel

[xal1983]

The transactional attribute currently allows you to choose your isolation level. However, CSLA's TransactionIsolationLevel enum does not include the option for Snapshot.

Describe the solution you'd like
Add Snapshot as an option to TransactionIsolationLevel. Also the new option needs to be handled in TransactionalDataPortal.GetIsolationLevel

Describe alternatives you've considered
I looked for ways to handle this without this change but the TransactionalDataPortal doesn't seem to be extendable or replaceable.

I would be more than happy to send a PR for this. Hopefully this can be included in a CSLA 9 update!

[rockfordlhotka]

I think this makes sense @xal1983 , please do submit PRs for v9 and into main.