Add link_to_subnet and link_to_pod_subnet for aks deployments

[nricciar]

The changes in this PR are as follows:

• adds link_to_subnet to agent pool config
• adds link_to_pod_subnet to agent pool config

I have read the contributing guidelines and have completed the following:

• Tested my code end-to-end against a live Azure subscription.
• Updated the documentation in the docs folder for the affected changes.
• Written unit tests against the modified code that I have made.
• Updated the release notes with a new entry for this PR.
• Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

#r "nuget:Farmer"

open System
open System.IO
open Farmer
open Farmer.Arm.ContainerService
open Farmer.Builders
open Farmer.ContainerService

type AksDeploymentRequestV1 =
    { ManagementResourceGroupName: string
      TenantMsi: UserAssignedIdentityConfig
      PodSubnet: ResourceId
      NodeSubnet: ResourceId }

type KubenetBuilder() =
    inherit NetworkProfileBuilder()

    member _.Yield = {
        NetworkPlugin = Some ContainerService.NetworkPlugin.AzureCni
        LoadBalancerSku = None
        DnsServiceIP = None
        DockerBridgeCidr = None
        ServiceCidr = None
    }

let aksResourceV1 (req: AksDeploymentRequestV1) =
    let networkProfile = KubenetBuilder()
    aks {
        name $"{req.ManagementResourceGroupName}-aks"
        tier Tier.Standard
        service_principal_use_msi
        add_identity req.TenantMsi
        kubelet_identity req.TenantMsi
        network_profile networkProfile.Yield
        enable_workload_identity
        enable_image_cleaner
        enable_private_cluster
        dns_prefix "aks"
        add_agent_pools
            [ agentPool {
                  name "systempool"
                  count 2
                  disk_size 128<Gb>
                  add_availability_zones [ "1"; "2"; "3" ]
                  vm_size (Vm.CustomImage "Standard_D2s_v3")
                  link_to_subnet req.NodeSubnet
                  link_to_pod_subnet req.PodSubnet
              }
              agentPool {
                  name "userpool"
                  user_mode
                  disk_size 128<Gb>
                  add_availability_zones [ "1"; "2"; "3" ]
                  enable_autoscale
                  autoscale_min_count 2
                  autoscale_max_count 4
                  vm_size (Vm.CustomImage "Standard_D4s_v3")
                  link_to_subnet req.NodeSubnet
                  link_to_pod_subnet req.PodSubnet
              } ]
    }

let msi = userAssignedIdentity { name "aks-rg-msi" }
let aksDeploy = 
    { ManagementResourceGroupName = "aks-rg"
      TenantMsi = msi
      PodSubnet = Arm.Network.subnets.resourceId (ResourceName "aks-rg", ResourceName "aksPod" )
      NodeSubnet = Arm.Network.subnets.resourceId (ResourceName "aks-rg", ResourceName "aksNode" ) }

arm {
    location Location.EastUS2
    add_resources [
        msi
        aksResourceV1 aksDeploy
    ]
}
|> Writer.quickWrite "aks-on-vnet"

[ninjarobot]

You should replace the existing SubnetName and PodSubnetName with these new properties rather than duplicating them. Otherwise both can be set and it's not clear from the record what will be happen (people will have to reverse engineer the code to see SubnetName takes precedence over Subnet).