Control canonical links in load_stac_from_job

[Silvren]

Summary

• add a keyword-only canonical_link mode to Connection.load_stac_from_job
• preserve current behavior with "auto": prefer a canonical link and fall back to the regular results URL
• support "require" to fail fast when no signed canonical link can be obtained
• support "avoid" to use the regular results URL without fetching result metadata
• validate unsupported mode values and document the new API

Testing

• python -m pytest tests/rest/test_connection.py::TestLoadStac -q (65 passed)
• pre-commit run --files CHANGELOG.md openeo/rest/connection.py tests/rest/test_connection.py

Fixes #634