[VL] RAS: Cost model file

Currently cost models are implemented as Scala code which requires compilation before used. We'd add support for a simple DSL (or leveraging JSON / YAML etc.) to make it easier to tune against the cost model by constructing it from an external cost model file.

In the initial version, we can just make the file store some magical / empirical numeric costs for different operators. As we haven't yet introduced Spark's statistics into RAS planner.