dynamic_planning/join_filter_pushdown env var resolution lives outside _opt()

Part of #23740.

`dynamic_planning` and `join_filter_pushdown` are `_opt("executor")` with no `env_var`
argument, so `StreamingOptions` never reads `CUDF_POLARS__EXECUTOR__DYNAMIC_PLANNING`/
`..._JOIN_FILTER_PUSHDOWN` itself. That check instead lives in a separate, hand-rolled
block in `ConfigOptions.from_polars_engine`, which branches on whether the key is present
in the executor_options dict at all. Give both fields a real `env_var` and `default` on
`_opt()`, matching every other field, and delete the ad hoc block. Both then always
resolve to a concrete `DynamicPlanningOptions | None`/`JoinFilterPushdownOptions | None`
instead of `UNSPECIFIED`.