Issue94 extend stac collection

[VincentVerelst]

• For now a user has to provide the location of the STAC collection they want to extend. Possible to later add a feature that automatically searches for an existing STAC collection
• The user also has to make sure they use a different job_tracker than the one already existing, otherwise new jobs will not be run. This is logged here: https://github.com/Open-EO/openeo-python-client/issues/558
• Also updated the metadata of the ground truth dataset, such that it can be read by load_stac in openeo