[Feature] Update IAM auth to warn that user field is ignored

[colin-rogers-dbt]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt-redshift functionality, rather than a Big Idea better suited to a discussion

Describe the feature

As seen in #845

Someone can reasonably change their profile to use IAM auth and we still allow them to specify a user field despite redshift resolving to a different database user at runtime. This can result in confusion where someone thinks they are using a different database user than they actually are.

We should update this section to not pass this to redshift at all and instead add a warning that the field is being ignored.

Describe alternatives you've considered

Explicitly not allowing user to be passed with IAM auth method (could create a breaking change)

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response

[adam-bloom]

I'm confused here - we currently use this field, and it behaves as expected. In fact, our IAM auth policies require that the connections are initiated with a given db_user field. Is this a difference between serverless redshift and regular redshift? Is there another way we'd be able to pass the user if this is removed?