Executor Plugin API

It would be nice if we could specify an arbitrary class to run within each executor for debugging and instrumentation. Its hard to do this currently because:

a) you have no idea when executors will come and go with DynamicAllocation, so don't have a chance to run custom code before the first task
b) even with static allocation, you'd have to change the code of your spark app itself to run a special task to "install" the plugin, which is often tough in production cases when those maintaining regularly running applications might not even know how to make changes to the application.

For example, https://github.com/squito/spark-memory could be used in a debugging context to understand memory use, just by re-running an application with extra command line arguments (as opposed to rebuilding spark).

I think one tricky part here is just deciding the api, and how its versioned. Does it just get created when the executor starts, and thats it? Or does it get more specific events, like task start, task end, etc? Would we ever add more events? It should definitely be a DeveloperApi, so breaking compatibility would be allowed ... but still should be avoided. We could create a base class that has no-op implementations, or explicitly version everything.

Note that this is not needed in the driver as we already have SparkListeners (even if you don't care about the SparkListenerEvents and just want to inspect objects in the JVM, its still good enough).