adding IAM role authentication support for redshift

[abbywh]

resolves #623
docs dbt-labs/docs.getdbt.com/#

Problem

dbt-redshift currently does not natively support the redshift authentication method get-cluster-credentials-with-iam. A dbt customer using IAM roles as db users currently has no way to authenticate as those users.

Solution

There is now a new redshift connection method to authenticate as an IAM role, called IAMR. This strongly resembles the classic IAM authentication, except the user field is unnecessary.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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[abbywh]

closing dupe