[SPARK-15703][SCHEDULER][CORE][WEBUI] Make ListenerBus event queue size configurable

## What changes were proposed in this pull request?
This change adds a new configuration entry to specify the size of the spark listener bus event queue. The value for this config ("spark.scheduler.listenerbus.eventqueue.size") is set to a default to 10000.

Note:
I haven't currently documented the configuration entry. We can decide whether it would be appropriate to make it a public configuration or keep it as an undocumented one. Refer JIRA for more details.

## How was this patch tested?
Ran existing jobs and verified the event queue size with debug logs and from the Spark WebUI Environment tab.

Author: Dhruve Ashar <dhruveashar@gmail.com>

Closes #14269 from dhruve/bug/SPARK-15703.