Expose `configureTaskDefault` on the extension

[Vampire]

I'm currently trying to set up a second pitest task to run with descartes mutation engine too in the same Gradle invocation.

There are quite some properties to configure, but the most I want to be setup just like the built-in pitest task wired to the extension.

You should not just configure all Pitest tasks as that is against best-practice for reusability,
but it would be nice if you expose the configureTaskDefault logic as a method in the extension.

I'm now leveraging that the plugin is written in Groovy and do tasks.registering(PitestTask::class) { project.plugins.findPlugin(PitestPlugin::class)!!.invokeMethod("configureTaskDefault", this) } to call the private method and do some customization afterwards, but that is not too nice of course. :-D