Provide a pluggable executor for tasks

[rtyler]

I'm thinking a lot more about how scripts are invoked on machines. Perhaps rather than providing transport-level privilege escalation via #2, instead the "shell executor" shouldn't just be a dumb /bin/sh but rather something configurable at run time.

• ShellExecutor (/bin/sh scriptfile)
• SudoExecutor (sudo scriptfile)
• JailShell ([sudo] jexec scriptfile)

Each of these should probably have some form of options, but I don't believe they need to necessarily be options specified at task time 🤔

(thinking out loud)

Perhaps a plan should be able to define the executor and configuration for it. For example, if I have a plan that sets up PostgreSQL in a jail, the plan would specify the details about the jail for jexec.