use IAM auth for Redshift

[drewbanin]

We can use boto3's get_cluster_credentials to get temp creds, then auth with those

[danielchalef]

I'd be happy to contribute this. Would you recommend I subclass and extend RedshiftAdapter or modify RedshiftAdapter?

[drewbanin]

hey @danielchalef - that's great! I would just modify the RedshiftAdapter. We have different methods for authing with BigQuery -- you may be able to use some of that code as an example.

Happy to help here however I can!

[danielchalef]

See PR #769 . Let me know if you'd like any mods. I noticed that the PR did not pass a codeclimate check.

[drewbanin]

Just merged #818 to add support for this feature. Thanks for the suggestion and the initial implementation @danielchalef :)