This is a developer facing function that is only used if you are creating
your own blueprint subclass. It is called from forge() and dispatches off
the S3 class of the blueprint. This gives you an opportunity to forge the
new data in a way that is specific to your blueprint.
run_forge() is always called from forge() with the same arguments, unlike
run_mold(), because there aren't different interfaces for calling
forge(). run_forge() is always called as:
run_forge(blueprint, new_data = new_data, outcomes = outcomes)
If you write a blueprint subclass for new_xy_blueprint(),
new_recipe_blueprint(), new_formula_blueprint(), or new_blueprint(),
then your run_forge() method signature must match this.