Allow private ip for cloudsql auth integration

[KyriosGN0]

Is your feature request related to a problem? Please describe.

We want to use Vault to generate credentials for our cloudsql (postgres) but we can't use it when the cloudsql instances are only available with Private IP (even if vault is in the VPC itself)

Describe the solution you'd like
connect to cloudsql postgres instances with IAM hosed only with private ip

Describe alternatives you've considered
currently we use static credentials we store in vault.