Skip jobs with unchanged dependencies

[reynoldsbd]

This feature has two parts:

1. Allow individual jobs to report an unchanged outcome
2. If all of a job's dependencies reported unchanged, then skip execution of that job and set its outcome to unchanged as well

For part 1, create a new enum:

#[non_exhaustive] // so we can add more outcomes in the future
pub enum Outcome {
    Success,
    Unchanged,
}

Change return type of Job::run to Result<Outcome, Self::Error>

Update JobState::Succeeded variant to include outcome value

For part 2, update Schedule::mark_complete to accept the full Result of job run and store job outcome appropriately. Also add logic to recursively set unchanged outcome for dependent jobs

[reynoldsbd]

Open question: Does this need to be configurable?

I don't think so. If an implementation of Job wishes to avoid this behavior it can simply choose not to return Outcome::Unchanged.