Added new data source azurerm_network_ddos_protection_plan

[tracypholmes]

Fixes #3851.

Output from acceptance testing

TF_ACC=1 go test ./azurerm -v -parallel 11 -test.run=TestAccAzureRMNetworkDDoSProtectionPlan -timeout 180m -ldflags="-X=github.com/terraform-providers/terraform-provider-azurerm/version.ProviderVersion=acc"
=== RUN   TestAccAzureRMNetworkDDoSProtectionPlan
=== RUN   TestAccAzureRMNetworkDDoSProtectionPlan/normal
=== RUN   TestAccAzureRMNetworkDDoSProtectionPlan/normal/basic
=== RUN   TestAccAzureRMNetworkDDoSProtectionPlan/datasource
=== RUN   TestAccAzureRMNetworkDDoSProtectionPlan/datasource/basic
--- PASS: TestAccAzureRMNetworkDDoSProtectionPlan (184.15s)
    --- PASS: TestAccAzureRMNetworkDDoSProtectionPlan/normal (88.13s)
        --- PASS: TestAccAzureRMNetworkDDoSProtectionPlan/normal/basic (88.13s)
    --- PASS: TestAccAzureRMNetworkDDoSProtectionPlan/datasource (96.01s)
        --- PASS: TestAccAzureRMNetworkDDoSProtectionPlan/datasource/basic (96.01s)
PASS
ok      github.com/terraform-providers/terraform-provider-azurerm/azurerm    184.222s

[katbyte]

Thanks for the PR @tracypholmes ! Overall this looks good. Once the few minor comments i've left inline are addressed this should be good to merge!

[ghost]

This has been released in version 1.34.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 1.34.0"
}
# ... other configuration ...

[ghost]

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