Support common external nuisance regressors in decision tree

[tsalo]

We can accept any nuisance regressors we want as long as they're in BIDS datasets, including ones from outside fMRIPrep (e.g., PhysioPrep, fMRIPost-phase).

Basically, fMRIPost-tedana will need some way to aggregate confounds across input datasets. The only thing I need to figure out is if we can just use the tedana decision tree config for this collection, or if we need something like XCP-D's new confound config format.