Refactor common executable code into an otto-exec crate

[rtyler]

The sh step, local-orchestrator, and agent all have redundant code for executing and then streaming data from a subprocess.

This should get refactored into a common crate

[rtyler]

This code is the os_pipe using code that executes a subprocess and streams its stdout and stderr

[sladyn98]

@rtyler Hey is this something that a person trying to learn rust can pick up or is there something easier :)

[rtyler]

@sladyn98 sorry for the delay, I've been incredibly busy with work lately and haven't had much time to work on Otto much less anything else 😦

I think this should be a relatively straight forward learning exercise, but I probably wouldn't start with it as a first taste of Rust. Depending on your timezone overlap, if you ping me over the weekend in the #otto channel on Freenode I can try to help show you around. 😄