Migrate `Plan` to `RelOperator`

[leiysky]

Background

Plan is a legacy design for planner v2 to be adapted with Interpreters. But in fact Interpreter is not a good abstraction.

In the ideal world, any statement can be optimized by optimizer and be expressed with a PhysicalPlan thus can be send to any node in the cluster to execute.

To make this work smoothly, we can migrate the Plans to RelOperator one by one, and introduce a universal Interpreter to execute the plans that can be expressed with a single SExpr(just like query execution in current implementation).

Tracking

• Migrating delete plan Unify execution of DML statements and queries #14482

[leiysky]

@BohuTANG PTAL

[BohuTANG]

@zhang2014 looks need this.