add `command` support

Right now we construct the `command` to run under the hood using `python {script} {script_args}`, where {script_args} are the kwargs dict converted to string flags. We should add optional `command` flag that lets user just specify the full command themselves. In this case, script args passed to kwargs are ignored, and the command runs as specified.

No files or tests are named in the issue. Locate the code that constructs the current `python {script} {script_args}` command, then trace how kwargs become script flags. Done means an optional `command` value runs unchanged while kwargs-derived script arguments are ignored; otherwise the existing command behavior remains intact.

Written by the indexing model from the issue text.