Add ability to auto-cancel a run

[naegelejd]

We found a use case where we'd like to stream to/from Tyger using tyger run exec, but also have the ability to cancel the run if necessary. This can be accomplished by parsing the output of tyger run exec to learn the run_id, then issuing a cancellation command, but it could also happen automatically.

For example, tyger run exec --cancel-on-exit could fully cancel and reap the run if the process is killed by interrupt.