Allow specification of `snakebids` config file

With the potential ballooning in size of snakebids apps CLIs, plus the difficulty of setting `pybids` filters via CLI, it would be useful to be able to specify a config file to be passed to the app (similar to `fMRIPrep`'s `--config-file`. This is especially relevant for dockerized apps, where the shipped config cannot be directly modified with any ease.

Probably this can directly use `--configfile` from Snakebids, although I'd need to look into exactly how this would work. I'll post more details as they come.

Start by reviewing Snakebids' CLI handling and how Snakemake's --configfile option could be exposed. Define the behavior for passing a config file to an app, including dockerized use, and verify that the resulting CLI supports the requested workflow.

Written by the indexing model from the issue text.