Support delete for plan2 (#2805)

pkg/frontend/mysql_cmd_executor.go

pkg/frontend/session.go

pkg/pb/plan/plan.pb.go

pkg/sql/colexec2/deletion/deletion.go

pkg/sql/colexec2/deletion/types.go

pkg/sql/compile2/compile.go

pkg/sql/compile2/operator.go

pkg/sql/compile2/types.go

pkg/sql/plan2/build_delete.go

proto/plan.proto