add ddos_protection_plan configuration to vnet

[jeongkyeong]

Modify vnet's ddos protection plan to be configurable.
The default value is 'null', so set it only when it is necessary.

Input variable example

module "vnet" {
  source              = "Azure/vnet/azurerm"
  
  ...
  ddos_protection_plan = {
      enable = true
      id     = azurerm_network_ddos_protection_plan.ddos_protection_plan.id
  }
}

Fixes #000

Changes proposed in the pull request:

[ghost]

All CLA requirements met.

[ianychoi]

Tagging @lonegunmanb to ask for review (who reviewed last several PRs).

@jeongkyeong: would you describe how it would be applicable by specifying use case(s) you are on mind?

Note that I am not an authorized reviewer regarding your PR but hope it helps.

[jeongkyeong]

Tagging @lonegunmanb to ask for review (who reviewed last several PRs).

@jeongkyeong: would you describe how it would be applicable by specifying use case(s) you are on mind?

Note that I am not an authorized reviewer regarding your PR but hope it helps.

Added one of the features of vnet, which allows you to set up a ddos protection plan. And this feature is also officially supported by Terraform.

This is a use case in terraform code(which is also being used as a module) using the azure vnet module.

variable "ddos_protection_plan" {
  type    = bool
  default = false
}

resource "azurerm_network_ddos_protection_plan" "ddos_protection_plan" {
  count               = var.ddos_protection_plan ? 1 : 0
  name                = "ddos-protection-${var.name}"
  location            = azurerm_resource_group.rg.location
  resource_group_name = azurerm_resource_group.rg.name
}

module "vnet" {
  source              = "Azure/vnet/azurerm"
  vnet_name           = var.name
  resource_group_name = azurerm_resource_group.rg.name
  address_space       = [var.vpc_address_space]
  subnet_prefixes     = [var.public_subnet_cidr, var.private_subnet_cidr]
  subnet_names        = var.subnet_names

  tags = local.tags

  ddos_protection_plan = var.ddos_protection_plan ? {
    enable = var.ddos_protection_plan
    id     = azurerm_network_ddos_protection_plan.ddos_protection_plan[0].id
  } : null
}

[lonegunmanb]

LGTM!