load_stac: support unsigned "own" job results from post_dry_run phase

[soxofaan]

spin-off from #1602:

The wet run phase of load_stac supports loading "own" job results from unsigned /jobs/{job_id}/results URLs,
but that is not yet properly supported in the "post dry run" phase where the STAC resource now also has to be traversed to determine the "global extent"

some side-effects like non-fatal warnings can be observed from tests like tests.test_api_result.TestLoadStac.test_load_stac_from_unsigned_partial_job_results_basic:

WARNING openeogeotrellis.backend:backend.py:1332 post_dry_run failed: Failed to extract spatial extent from source_id=('load_stac', ('https://openeo.test/openeo/1.1/jobs/j-2405078f40904a0b85cf8dc5dd55b07e/results', (), ())) with constraint={}: e=ConnectionError(MaxRetryError('HTTPSConnectionPool(host='openeo.test', port=443): Max retries exceeded with url: /openeo/1.1/jobs/j-2405078f40904a0b85cf8dc5dd55b07e/results (Caused by NameResolutionError("HTTPSConnection(host='openeo.test', port=443): Failed to resolve 'openeo.test' ([Errno -2] Name or service not known)"))'))