Introduce the component system for managing pre-installed addons

* Previously: Typhoon provisions clusters with kube-system components
like CoreDNS, kube-proxy, and a chosen CNI provider (among flannel,
Calico, or Cilium) pre-installed. This is convenient since clusters
come with "batteries included". But it also means upgrading these
components is generally done in lock-step, by upgrading to a new
Typhoon / Kubernetes release
* It can be valuable to manage these components with a separate
plan/apply process or through automations and deploy systems. For
example, this allows managing CoreDNS separately from the cluster's
lifecycle.
* These "components" will continue to be pre-installed by default,
but a new `components` variable allows them to be disabled and
managed as "addons", components you apply after cluster creation
and manage on a rolling basis. For some of these, we may provide
Terraform modules to aide in managing these components.

```
module "cluster" {
  # defaults
  components = {
    enable = true
    coredns = {
      enable = true
    }
    kube_proxy = {
      enable = true
    }
    # Only the CNI set in var.networking will be installed
    flannel = {
      enable = true
    }
    calico = {
      enable = true
    }
    cilium = {
      enable = true
    }
  }
}
```

An earlier variable `install_container_networking = true/false` has
been removed, since it can now be achieved with this more extensible
and general components mechanism by setting the chosen networking
provider enable field to false.

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

@@ -1,11 +1,11 @@
 # Kubernetes assets (kubeconfig, manifests)
 module "bootstrap" {
-  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=452bcf379d94f5d479c1dee281fd479872271415"
+  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=a54fe54d9895f5dd51332b79533143f52792090f"
 
   cluster_name          = var.cluster_name
   api_servers           = [format("%s.%s", var.cluster_name, var.dns_zone)]
   etcd_servers          = [for fqdn in google_dns_record_set.etcds.*.name : trimsuffix(fqdn, ".")]
-  networking            = var.install_container_networking ? var.networking : "none"
+  networking            = var.networking
   network_mtu           = 1440
   pod_cidr              = var.pod_cidr
   service_cidr          = var.service_cidr

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

@@ -94,12 +94,6 @@ 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"
@@ -157,6 +151,9 @@ variable "components" {
     enable     = optional(bool)
     coredns    = optional(map(any))
     kube_proxy = optional(map(any))
+    flannel    = optional(map(any))
+    calico     = optional(map(any))
+    cilium     = optional(map(any))
   })
   default = null
 }