Feature: Add support for IAM auth via aws_postgres

[arvindth]

We got support for aws_postgres dbtype in spiffe/spire#4828. However, there's no current way to pass this via the helm chart.

Could we either add direct support for aws_postgres, or allow plugin_data's dbtype and connection_string to be respected until we do?

[kfox1111]

The devil in the details is going to be around how IAM auth works in k8s?

How does the spire-server pod get the credentials?

[kfox1111]

This depends on spiffe/spire#5495

[kfox1111]

Can you please try #464 in a test environment and see if it works for you? I don't have a way to test it.