EXPLAIN CODEGEN does not work with AQE

`EXPLAIN CODEGEN <query>` (and Dataset.explain("codegen")) prints out the generated code for each stage of plan. The current implementation is to match `WholeStageCodegenExec` operator in query plan and prints out generated code (https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/debug/package.scala#L111-L118 ). This does not work with AQE as we wrap the whole query plan inside `AdaptiveSparkPlanExec` and do not run whole stage code-gen physical plan rule eagerly (`CollapseCodegenStages`). This introduces unexpected behavior change for EXPLAIN query (and Dataset.explain), as we enable AQE by default now.