Decouple PlanExecutor from JSON plan generation

PlanExecutor has a method getOptimizerPlanAsJSON() that is used by DataSet environments to get a JSON version of the execution plan. To ease future work and to make it more maintainable we should get rid of that method and instead have a dedicated utility for generating JSON plans that the environments can use.

(The only reason this method is on the executor is because only flink-clients via flink-optimizer has the required components to derive a JSON plan.)