feat(vm): add full support for custom cloud-init disks

[ForsakenHarmony]

Contributor's Note

• I have added / updated documentation in /docs for any user-facing features or additions.
• I have added / updated templates in /example for any new or updated resources / data sources.
• I have ran make example to verify that the change works as expected.

Proof of Work

Have already used this in production, but I need to test the additional changes I made for this PR.

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[bpg]

Hi @ForsakenHarmony 👋🏼

Thank you for the PR! It's nice to simplify and cleanup the cloud-init handling.
But the new code causes some issue with cloned VM.
make example fails on applying resource_virtual_environment_vm.tf with

proxmox_virtual_environment_vm.trunks-example: Creation complete after 47s [id=103]
╷
│ Error: failed to add cloud-init device: invalid initialization interface type "" (should be one of: ide[n],sata[n],scsi[n])
│ 
│   with proxmox_virtual_environment_vm.data_vm,
│   on resource_virtual_environment_vm.tf line 207, in resource "proxmox_virtual_environment_vm" "data_vm":
│  207: resource "proxmox_virtual_environment_vm" "data_vm" {
│ 
╵

Also, this trivial acceptance test fails (with a different error tho)

func TestAccResourceVMCloudInit(t *testing.T) {
	t.Parallel()

	providerConfig := getProviderConfig(t)

	tests := []struct {
		name  string
		steps []resource.TestStep
	}{
		{"clone template", []resource.TestStep{
			{
				Config: providerConfig + `
				resource "proxmox_virtual_environment_vm" "test_cloudinit1_template" {
					node_name = "pve"
					started   = false
					name 	  = "test-cloudinit1-template"
					template  = "true"
					initialization {
						datastore_id = "local-lvm"
						interface    = "scsi4"
						ip_config {
							ipv4 {
								address = "dhcp"
							}
						}
					}
				}
				resource "proxmox_virtual_environment_vm" "test_cloudinit1" {
					node_name = "pve"
					started   = false
					name 	  = "test-cloudinit1"
					clone {
						vm_id = proxmox_virtual_environment_vm.test_cloudinit1_template.id
					}
					initialization {
						datastore_id = "local-lvm"
						interface    = "scsi4"
					    dns {
    						servers = ["1.1.1.1"]
    					}
						ip_config {
							ipv4 {
								address = "dhcp"
							}
						}
					}
				}
				`,
				Check: resource.ComposeTestCheckFunc(
				//TODO: check if the dns servers are set
				),
			},
			{
				RefreshState: true,
				Destroy:      false,
			},
		}},
	}

	accProviders := testAccMuxProviders(context.Background(), t)

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			t.Parallel()

			resource.Test(t, resource.TestCase{
				ProtoV6ProviderFactories: accProviders,
				Steps:                    tt.steps,
			})
		})
	}
}

Would you please take a look?