Save encoded execution plan in the ExecutionStage to reduce cost of task serialization and deserialization

[yahoNanJing]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Currently we save the execution plan by its decoded format. When scheduling tasks to executors, we encode them. When there're thousands of tasks for a stage, it will cost too much for the encoding process.

Describe the solution you'd like

It's better for us to just save the execution plan by its encoded format, since there are few places that we need the decoded execution plan in the scheduler.

Describe alternatives you've considered

Additional context