engine_args

first thought i wanted to rename `other_args` to something specific to `metab_Kmodel`, because that's silly to have such a generically named argument that applies to just one model.

but then maybe the potential_args can truly be populated with any available other_args for every model type? `engine_args` would sound cool...

also think about adding a control argument for bayes (stan, at least) models. would be nice to be able to set tree depth, etc.. this could probably go into `engine_args`.

No files, tests, or entry points are named. Trace where `other_args` and `potential_args` are defined and consumed for each model type, then inspect how Bayesian model controls are passed; done requires an agreed argument name and documented behavior for model-specific and Stan controls.

Written by the indexing model from the issue text.