IAM support

[OJFord]

The typical/recommended way to use Redshift is with IAM afaict, but this backend requires a username/password.

[shimizukawa]

I think it would be great if we could use IAM. However, I don't know how to implement it for now.
I suspect it could be achieved by changing the implementation to use the redshift_connector instead of psycopg.
And I believe this will be necessary for #167 and #168.