Add a intermediate resource to manage machines

[Freddo3000]

I'd like if there was a intermediate resource between machine and instance so a user can configure parameters unrelated to power parameters, such as domain, hostname, pool, etc, as well as configuring network interfaces that cannot otherwise be allocated using the maas_instance resource.

For example,

// Find a few machines and allocate them, set them to appropriate resource pools
resource "maas_allocation" "example" {
  count = 3
  // Parameters used for finding machines to allocate.
  // Machines found are changed to be "allocated" https://maas.io/docs/how-to-allocate-machines-with-maas
  system_id = maas_machine.example[count.index].system_id  // Or leave null
  min_memory = 12345
  min_cpu_count = 12
  tags = [...]
  zone = "some_AZ"

  // Parameters that change the machine parameters
  hostname = "my_new_hostname"
  domain = "a non-default domain"
  pool = "a non-default pool"

  mark = true // New: Mark machine as "Allocated" in MAAS
}

// Create a few VLAN interfaces, as these cannot be created after a machine has been deployed
resource "maas_network_interface_vlan" "example" {
  count = length(maas_allocation.example)
  depends_on = [maas_allocation.example]
  fabric = "fabric-1"
  machine = maas_llocation.example[count.index].system_id // or fqdn, or hostname
  parent = "ens1"

  vlan = 123
}

// Or block devices...
resource "maas_block_device" "example" {
  count = length(maas_allocation.example)
  depends_on = [maas_allocation.example]
  machine = maas_llocation.example[count.index].system_id // or fqdn, or hostname
  ...
}

// Deploy the allocated machines
resource "maas_instance" "example" {
  count = length(maas_allocation.example)
  depends_on = [maas_allocation.example, maas_network_interface_vlan.example, maas_block_device.example]

  allocate_params {
    // Machines marked as "Allocated" should only be able to be deployed by specifying the
    // host or system_id
    hostname = maas_allocation.example[count.index].hostname
    system_id = maas_allocation.example[count.index].system_id // Known from the previous allocation.
  }
}

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