[WIP][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).

The design doc can be found at [SPARK-3174](https://issues.apache.org/jira/browse/SPARK-3174) (under "Heuristics for Scaling Executors". This deviates from the design in two ways: (1) the remove policy is significantly simplified by cutting out the exponential increase logic that mirrors the add policy, and (2) the configuration of the add policy is split up into `addExecutorThreshold` and `addExecutorInterval`. The main reason for this is that the user may want to configure how quickly executors are added, but want to keep the trigger condition the same.

Comments and feedback are most welcome (but please keep in mind that this is still WIP!).
The number of configuration options here is a little scary.  Do we need all of these things to be configurable?  The first 3 seem like things we definitely need.  Some of the remaining things seem very tied to how long it takes a new executor to start up, and I wonder if we could hardcode them for now, and make them configurable later only if people find the defaults to be problematic.