[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.
@pwendell is there a reason not to include units?  I'd argue they should all include units...I'm certainly not a major user and still have spent too long searching the docs to try to figure out what the units for something are.  I think the tiny amount of additional time to type a longer parameter name has a huge benefit in terms of readability down the road?