Add an install_container_networking variable (default true)

* When `true`, the chosen container `networking` provider is installed during cluster bootstrap
* Set `false` to self-manage the container networking provider. This allows flannel, Calico, or Cilium
to be managed via Terraform (like any other Kubernetes resources). Nodes will be NotReady until you
apply the self-managed container networking provider. This may become the default in future.

google-cloud/flatcar-linux/kubernetes/variables.tf

@@ -94,6 +94,12 @@ variable "networking" {
   default     = "cilium"
 }
 
+variable "install_container_networking" {
+  type        = bool
+  description = "Install the chosen networking provider during cluster bootstrap (use false to self-manage)"
+  default     = true
+}
+
 variable "pod_cidr" {
   type        = string
   description = "CIDR IPv4 range to assign Kubernetes pods"