Feature request: genrule with directory output / run_binary with command input

I'm interested in a rule that takes a shell command string as input like `genrule` but supports directories as outputs like `run_binary`. Would the maintainers of this library be receptive to a PR adding one?

My intuition would be to add a rule `run_shell` that shares a lot of the code of `run_binary`. But I can also imagine just adding a parameter to `run_binary` (incompatible with `tool`).

Yes, one can just use `run_binary` with `sh_binary`, but that requires to targets instead of one, plus a `.sh` file (or a third `write_file` target). If your script is more than a few lines, that's probably the better approach anyways, but it feels like a lot of boilerplate when I just want to do a simple transformation over an indeterminate number of files.

Read the existing genrule and run_binary implementations first, then compare the proposed run_shell rule with the alternatives using sh_binary and write_file. Clarify whether the API should be a new rule or a run_binary parameter, and whether tool support is excluded. Done means a shell-command rule can produce directory outputs without the extra target and script boilerplate.

Written by the indexing model from the issue text.