Add IPv6 support for Typhoon Azure clusters

* Define a dual-stack virtual network with both IPv4 and IPv6 private
address space. Change `host_cidr` variable (string) to a `network_cidr`
variable (object) with "ipv4" and "ipv6" fields that list CIDR strings.
* Define dual-stack controller and worker subnets. Disable Azure
default outbound access (a deprecated fallback mechanism)
* Enable dual-stack load balancing to Kubernetes Ingress by adding
a public IPv6 frontend IP and LB rule to the load balancer.
* Enable worker outbound IPv6 connectivity through load balancer
SNAT by adding an IPv6 frontend IP and outbound rule
* Configure controller nodes with a public IPv6 address to provide
direct outbound IPv6 connectivity
* Add an IPv6 worker backend pool. Azure requires separate IPv4 and
IPv6 backend pools, though the health probe can be shared
* Extend network security group rules for IPv6 source/destinations

Checklist:

Access to controller and worker nodes via IPv6 addresses:

  * SSH access to controller nodes via public IPv6 address
  * SSH access to worker nodes via (private) IPv6 address (via
    controller)

Outbound IPv6 connectivity from controller and worker nodes:

```
nc -6 -zv ipv6.google.com 80
Ncat: Version 7.94 ( https://nmap.org/ncat )
Ncat: Connected to [2607:f8b0:4001:c16::66]:80.
Ncat: 0 bytes sent, 0 bytes received in 0.02 seconds.
```

Serve Ingress traffic via IPv4 or IPv6 just requires setting
up A and AAAA records and running the ingress controller with
`hostNetwork: true` since, hostPort only forwards IPv4 traffic

docs/addons/ingress.md

@@ -37,7 +37,7 @@ resource "google_dns_record_set" "some-application" {
 
 ## Azure
 
-On Azure, a load balancer distributes traffic across a backend address pool of worker nodes running an Ingress controller deployment. Security group rules allow traffic to ports 80 and 443. Health probes ensure only workers with a healthy Ingress controller receive traffic.
+On Azure, an Azure Load Balancer distributes IPv4/IPv6 traffic across backend address pools of worker nodes running an Ingress controller deployment. Security group rules allow traffic to ports 80 and 443. Health probes ensure only workers with a healthy Ingress controller receive traffic.
 
 Create the Ingress controller deployment, service, RBAC roles, RBAC bindings, and namespace.
 
@@ -53,10 +53,10 @@ app2.example.com -> 11.22.33.44
 app3.example.com -> 11.22.33.44
 ```
 
-Find the load balancer's IPv4 address with the Azure console or use the Typhoon module's output `ingress_static_ipv4`. For example, you might use Terraform to manage a Google Cloud DNS record:
+Find the load balancer's addresses with the Azure console or use the Typhoon module's outputs `ingress_static_ipv4` or `ingress_static_ipv6`. For example, you might use Terraform to manage a Google Cloud DNS record:
 
 ```tf
-resource "google_dns_record_set" "some-application" {
+resource "google_dns_record_set" "app-record-a" {
   # DNS zone name
   managed_zone = "example-zone"
 
@@ -66,6 +66,17 @@ resource "google_dns_record_set" "some-application" {
   ttl     = 300
   rrdatas = [module.ramius.ingress_static_ipv4]
 }
+
+resource "google_dns_record_set" "app-record-aaaa" {
+  # DNS zone name
+  managed_zone = "example-zone"
+
+  # DNS record
+  name    = "app.example.com."
+  type    = "AAAA"
+  ttl     = 300
+  rrdatas = [module.ramius.ingress_static_ipv6]
+}
 ```
 
 ## Bare-Metal