feat: add harvester_cloudinit_secret

[FrankYang0529]

Issue: harvester/harvester#2958

Test plan

Setup
Follow PR Test Env Setup

Case 1: Apply same terraform doesn't trigger change

1. Create a file main.tf.

resource "harvester_cloudinit_secret" "ex1" {
  name             = "ex1"
  namespace        = "default"
  user_data_base64 = "I2Nsb3VkLWNvbmZpZwpwYWNrYWdlX3VwZGF0ZTogdHJ1ZQpwYWNrYWdlczoKICAtIHFlbXUtZ3Vlc3QtYWdlbnQKcnVuY21kOgogIC0gLSBzeXN0ZW1jdGwKICAgIC0gZW5hYmxlCiAgICAtIC0tbm93CiAgICAtIHFlbXUtZ3Vlc3QtYWdlbnQuc2VydmljZQpwYXNzd29yZDogdGVzdApjaHBhc3N3ZDoKICBleHBpcmU6IGZhbHNlCnNzaF9wd2F1dGg6IHRydWUK"
}

resource "harvester_cloudinit_secret" "ex2" {
    name = "ex2"
    namespace = "default"

    user_data    = <<-EOF
    #cloud-config
    password: test
    chpasswd:
      expire: false
    ssh_pwauth: true
    package_update: true
    packages:
      - qemu-guest-agent
    runcmd:
      - - systemctl
        - enable
        - '--now'
        - qemu-guest-agent
    EOF
    network_data = ""
}

2. Run terraform apply. Terraform should add ex1 and ex2 secrets to the cluster.
3. Run terraform apply again. Nothing change.

Case 2: Change content to empty will update the secret

1. Follow case 1.
2. Update the file main.tf.

resource "harvester_cloudinit_secret" "ex1" {
  name             = "ex1"
  namespace        = "default"
  user_data_base64 = ""
}

resource "harvester_cloudinit_secret" "ex2" {
    name = "ex2"
    namespace = "default"

    user_data    = ""
    network_data = ""
}

3. Run terraform apply. The content of ex1 and ex2 secrets will be empty.
4. Run terraform apply again. Nothing change.

[futuretea]

Also need to consider another test case：

1. apply with user_data not empty
2. change user_data to empty
3. apply again

[FrankYang0529]

Also need to consider another test case：

1. apply with user_data not empty
2. change user_data to empty
3. apply again

Added test case 2. Thanks.

[iosifnicolae2]

Any idea when this PR will be merged?

[chrisho]

LGTM, the test cases are working fine, thanks.

[guangbochen]

LGTM, and verified that the testing case works.

[guangbochen]

@bk201 please take a look when u are back only; merge the PR to process the validation first, thanks.