Ensure that Network Watcher is 'Enabled'

Description:

Enable Network Watcher for Azure subscriptions.

Rationale:

Network diagnostic and visualization tools available with Network Watcher help users understand, diagnose, and gain insights to the network in Azure.

In Azure Console -

1. Open the Azure Portal and go to Network Watcher.
2. Under Logs, select NSG flow logs.
3. Select Create NSG flow log and step through the wizard prompts.
   Note: it is recommended to use Version 2 for more detailed logs.

In Terraform -

1. For each azurerm_network_security_group resource, create and configure an azurerm_network_watcher_flow_log resource.

References:
https://learn.microsoft.com/en-us/azure/network-watcher/network-watcher-nsg-flow-logging-overview
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/network_watcher_flow_log