Add options
- `stream-stdout-in-response`
- `stream-stdout-in-response-on-error`
- `stream-command-kill-grace-period-seconds`
to allow defining webhooks which dynamically stream large content back to the
requestor. This allows the creation of download endpoints from scripts, i.e.
running a `git archive` command or a database dump from a docker container,
without needing to buffer up the original.
