Allow mvn create-extension outside of quarkus

[ia3andy]

Description
In the guide, https://quarkus.io/guides/extension-authors-guide, the extension dev happen directly in Quarkus. When I try to use the maven create-extension, I get "Creating standalone extension projects is not supported yet. Only adding modules under and existing pom.xml file is supported. -> [Help 1]"

I think the plugin (and the guide) should allow (demonstrate) creating an extension from outside of the Quarkus codebase. This feature is going to become really important with the soon coming Quarkus Platform.

@stuartwdouglas has created such a repository in the mean time: https://github.com/stuartwdouglas/rhte-extension/tree/empty

[aloubyansky]

This is actually a duplicate of #3321, iiuc.