Guidance for generating an RDS IAM token using v2 SDK

### Describe the issue

Following on from #1157 it would be great if documentation could be provided to indicate how to obtain an RDS IAM token using the V2 SDK. In V1 this was achieved with `com.amazonaws.services.rds.auth.RdsIamAuthTokenGenerator` but this class does not appear to have an equivalent in V2 (that I could find).

### Links

[Existing V1 mechanism in the RDS documentation](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/UsingWithRDS.IAMDBAuth.Connecting.Java.html)