vpc: support subnets[].allowSubnets #61

cozystack/cozystack#4478 adds an optional allowSubnets field (a list of CIDR strings) to each entry of subnets in the VirtualPrivateCloud app. It is rendered into spec.allowSubnets of the kube-ovn Subnet, so a VM on the subnet can act as a VPN or router gateway for a remote network.

The provider models subnets by hand, so the field needs to be added in internal/provider/vpc_schema.go (resource and data source nested attributes, as an optional list of strings) and in the expand/flatten pair in internal/provider/vpc_model.go. Until then the field cannot be set from Terraform, and a subnet that declares it through the API has it dropped from state.

This should wait for the upstream PR to merge, and the api/apps/v1alpha1 pin can be bumped at the same time to pick up the new Subnet.AllowSubnets field.