Warn about unsupported output format options

[Silvren]

Summary

• compare save_result options with the format parameters advertised by /file_formats
• warn about unsupported option names while preserving them in the process graph for backend compatibility
• share the validation logic between raster and vector cubes
• keep format matching case-insensitive and skip option validation when a backend does not publish parameter metadata
• extend DummyBackend.setup_file_format to define format parameters in tests

Testing

• python -m pytest tests/rest/datacube/test_datacube100.py tests/rest/datacube/test_vectorcube.py -q (478 passed, 2 skipped)
• python -m pytest tests/rest/datacube/test_datacube100.py::test_save_result_format tests/rest/datacube/test_datacube100.py::test_save_result_format_options_warning tests/rest/datacube/test_vectorcube.py::test_save_result_format_options_warning -q
• pre-commit run --files CHANGELOG.md openeo/rest/_testing.py openeo/rest/connection.py openeo/rest/datacube.py openeo/rest/vectorcube.py tests/rest/datacube/test_datacube100.py tests/rest/datacube/test_vectorcube.py

Fixes #649