Deploy to existing Azure VNets

Azure environments can now provision into a VNet that already exists in the target Azure subscription instead of standing up a new one. Set config.existing_vnet_id to the VNet's Azure resource ID and config.vnet_cidr to a range inside it, and Ryvn carves its AKS subnets out of that range.

Most deployments don't need this — Ryvn provisions the full Azure stack, VNet included, whether the target is your own subscription or your customer's BYOC tenancy. This is for the cases where a network team needs to own the VNet themselves: a hub-and-spoke topology enforced centrally, ExpressRoute or a central firewall on the path, address-space allocations that have to come out of a corporate IPAM. They hand Ryvn a VNet and a CIDR; Ryvn drops in.