[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.
Man, this is a lot of extra code just to be able to keep methods private... sorry you had to go through that.

In Java-land here we use `com.google.common.annotations.VisibleForTesting` and just change the method/field's visibility. The annotation serves as a guide ("don't call this if you're not writing a test").