install/generate the extensions.xml as option for the lock

[maxandersen]

would be nice to have the .mvn/extensions.xml generated as an option at least when running with lock.

i.e. mvn io.mvnpm:locker-maven-plugin:0.0.6:lock -Dlockextension for example would create .mvn/extensions.xml if not present - and if possible add it to extensions.xml if not present inside the extensions.xml

[ia3andy]

That's a good idea @maxandersen