Ensure that custom domains are configured in Azure App Service

Custom domains are often used in Azure App Service for a number of reasons, including security. As an origin location, App Service apps can be configured for mutual TLS using a custom domain and origin certificate.

In Azure Console -

1. Open the Azure Portal and go to App Services.
2. Select the App service you wish to edit.
3. Select Custom domains under Settings.
4. Add a custom domain entry.

In Terraform -

1. For each azurerm_app_service resource, create an azurerm_app_service_custom_hostname_binding resource.
2. Reference the app_service_name to link it to the App Service.

References:
https://learn.microsoft.com/en-us/azure/app-service/configure-language-dotnet-framework
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/app_service
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/app_service_custom_hostname_binding