Add process-local environments for OWN_GIL contexts

Enable _with_env NIFs to work with OWN_GIL mode by dispatching
requests to the dedicated OWN_GIL thread.

Changes:
- Add request types for env variants (CALL/EVAL/EXEC/CREATE_LOCAL_ENV)
- Add local_env_ptr field to py_context_t for passing env resources
- Add execute functions for env variants in OWN_GIL thread
- Add dispatch functions for env variants
- Update _with_env NIFs to dispatch for OWN_GIL mode
- Update destructor to skip DECREF for OWN_GIL envs
- Add get_nif_ref/1 to py_context for direct NIF access
- Add local_env tests to py_owngil_features_SUITE

Author: benoitc