Allow extensions in settings.xml

Since 3.3.1, we are now allowed to specify extensions in the projectBaseDir/.mvn/extensions.xml. This is great on it's own, but I have a case where repo urls are externalized to ~/.m2/settings.xml and these urls require an extension. Right now, the url and the extension that goes along with it are managed in two completely different places.

Ideally, I would like to see the option to add an extension to our existing ~/.m2/settings.xml.