feat: configure virtual applications

[sebastianlolv]

Created a variable for settings virtual app path mappings for windows web apps. Virtual Application doesn't exist for Linux web apps.

There is an issue when importing existing windows web apps with virtual application set it will try to delete a set virtual application.

Please contribute if this is the wrong way or I'm missing something.

[hknutsen]

According to the Azure provider docs for resource azurerm_windows_web_app, there can be one or more virtual_application blocks: https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/windows_web_app#virtual_application-1

Thus:

• Variable virtual_application (singular) should be renamed to virtual_applications (plural).
• Variable virtual_applications should be of type list(object) (ref.).