Incorrect dependency check in AwsIamCredentialPlugin

The AwsIamCredentialPlugin checks for the existence of `AwsBasicCredentials` before deciding whether to continue initializing. It throws a helpful error saying the `software.amazon.awssdk:rds` artifact should be on the classpath. The issue is `AwsBasicCredentials` comes from `software.amazon.awssdk:auth`. Checking for it does not guarantee the correct dependency is in the classpath.
For the most part, this does not cause new issues because exceptions are thrown either way. The native image support, however, uses this same check and results in failed builds if the `software.amazon.awssdk:auth` is in the classpath without `software.amazon.awssdk:rds`.