Add support for Azure Key Vault

[johncollinson2001]

Azure Key Vault is not directly supported by Azure Backup / Data Protection Services.

Any solution will likely require a custom script to extract the contents of the Key Vault, encrypt it, and upload it to a storage account which is configured to be backed up by Azure Backup.

A mechanism to execute the Key Vault backup on a schedule would be helpful (e.g. through a Logic App, or an Azure Automation workflow).

It would be nice if this module could abstract the user away from the scripting and automation. The user should simply provide the relevant config (as per other types of resources that are directly supported), and the module will then create a dedicated storage account, a logic app/automation workflow, and fully manage the scheduled backup of Key Vault data.