Add skipIfSuccessful option to prevent unnecessary DAG reruns

[yohamta]

Currently, there's no easy way to prevent a DAG from running if it has already completed successfully since its last scheduled execution. This can lead to unnecessary resource consumption, especially for resource-intensive tasks.

We would like to add a new DAG-level flag called runOnlyOnSchedule skipIfSuccessful. When set to true, Dagu will automatically check the last successful run time against the defined schedule. If the DAG has already run successfully since the last scheduled time, the current run will be skipped.

Related discussion: https://github.com/dagu-org/dagu/discussions/708