Add integration tests for IAM User auth

[mikealfare]

resolves #760

Problem

We currently support IAM User auth but we don't currently have any tests that verify this.

Solution

• add required infrastructure (users, roles, permissions, etc.) in AWS to support tests
• add tests for Database method auth
• add tests for IAM User method auth using a profile (give redshift_connector a profile name in your AWS config file)
• add tests for IAM User method auth using direct args (give redshift_connector the values that normally live in your AWS config file)
• update test.env.example so that contributors know what to provide to test this in their environment
• clean up unit tests since (I got lost trying to find the applicable tests since they were all in "test_redshift_adapter")

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

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-redshift contributing guide.

[colin-rogers-dbt]

Successful run with the new workflow: https://github.com/dbt-labs/dbt-redshift/actions/runs/8762024735/job/24112742150