[SPARK-3795] Heuristics for dynamically scaling executors

This is part of a bigger effort to provide elastic scaling of executors within a Spark application ([SPARK-3174](https://issues.apache.org/jira/browse/SPARK-3174)). This PR does not provide any functionality by itself; it is a skeleton that is missing a mechanism to be added later in [SPARK-3822](https://issues.apache.org/jira/browse/SPARK-3822).

Comments and feedback are most welcome. For those of you reviewing this in detail, I highly recommend doing it through your favorite IDE instead of through the diff here.
Another thing to think about here is cancellations - what happens if a stage is started but it's cancelled before all of its tasks complete.