Add AppPortal reverse proxy access control helpers

[jmcte]

Summary

• add AppPortal AccessControl list/update helpers with JSON entry serialization
• fix ReverseProxy list handling and add create/update/delete helpers
• add request-contract coverage for AccessControl and ReverseProxy payloads

Validation

• python -m pytest tests/test_core_service_apps.py -q
• PYTHONPATH=tests python -m pytest -q --ignore=tests/integration --ignore=tests/test_add_s3_sync_task.py --ignore=tests/test_core_user.py --ignore=tests/test_syno_api.py
• python -m compileall -q synology_api/core_service_apps.py tests/test_core_service_apps.py
• git diff --check
• live DSM add/remove smoke test for a temporary reverse proxy entry; final temp count returned to 0

Closes #179