rdb: integrate PN provisiong mode to remove dependency with IPAM

[Mia-Cross]

Now that the provisioning mode of a private network is readable from the API, we can remove the dependency to IPAM that was introduced to check whether the endpoint was provisioned with IPAM.

EDIT: We still have to enforce the explicit choice between ip_net and enable_ipam in the configuration of private endpoints for RDB Instance and Read Replica. We will be able to remove it when Terraform allows us to use SetNew on nested fields, so that we can set the value of private_network.0.enable_ipam in the diff in a CustomizeDiffFunc.