[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.

This is work-in-progress because it is missing two things: (1) logic for retrying a request to add/remove executors if the request is not fulfilled, and (2) unit tests.

Comments and feedback are most welcome (but please keep in mind that this is still WIP!).
I'm following the guidelines here http://docs.scala-lang.org/style/declarations.html#procedure-syntax. We've started to enforce this through code reviews in other PRs. Maybe we should clearly document this in the Spark style guide too.