Simplify Workload Identity Federation for Cloud SQL workloads

[Workload identity federation](https://cloud.google.com/sql/docs/mysql/connect-kubernetes-engine) makes it possible to log in to the databases using the identity managed by K8s principals and an IAM service account. This is tricky to set up.

Consider ways that the operator could assist with the configuration. Perhaps the user specify an IAM principal in the AuthProxyWorkload, then the operator could automatically configure the K8s service accounts to apply the correct K8s principal to the pods where the AuthProxyWorkload is attached.

See https://github.com/GoogleCloudPlatform/cloud-sql-proxy-operator/issues/705