Add IAM Authentication to dbt-postgres

[christopherscholz]

closes #3798

What are you changing in this pull request and why?

This change is connected to dbt-labs/dbt-core#8187.

The PR adds the ability to use AWS IAM Authentication for dbt-postgres. This extends the options in the profiles.yml. The updates are documented in this PR.

Checklist

• Add versioning components, as described in Versioning Docs
• Add a note to the prerelease version Migration Guide
• Review the Content style guide and About versioning so my content adheres to these guidelines.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

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[runleonarun]

Hi there @christopherscholz you can @mention me when this is ready for review and I will triage it to a writer and devX duo.

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[runleonarun]

Removed this from 1.7 milestone added related issue to 1.8 for now

[matthewshaver]

The attached issue in the Core repo was closed on April 10th (due to the decoupling of adapters). There was not a new one attached and since this has been in draft for a little over a year, I am closing this as well.

If we need to revisit, we can absolutely re-open this.