Validate load_stac properties with STAC queryables

[Silvren]

Summary

• discover queryables advertised directly by a STAC object
• for STAC API collections, inspect Filter conformance at the root and derive the collection queryables endpoint
• reuse the already loaded PySTAC object for cube metadata instead of fetching the STAC document twice
• apply the existing soft validation and warning behavior to unsupported load_stac property filters

Testing

• python -m pytest tests/test_metadata.py tests/rest/datacube/test_datacube100.py tests/rest/test_connection.py::TestLoadStac -q (569 passed, 2 skipped)
• pre-commit run --files CHANGELOG.md openeo/metadata.py openeo/rest/datacube.py tests/rest/datacube/test_datacube100.py tests/rest/test_connection.py

Fixes #884