i.sentinel.download: fix authentication

[landam]

i.sentinel.download fails with

Traceback (most recent call last):
  File "/home/landamar/.grass8/addons/scripts/i.sentinel.download", line 535, in <module>
    sys.exit(main())
             ~~~~^^
  File "/home/landamar/.grass8/addons/scripts/i.sentinel.download", line 406, in main
    dag._plugins_manager.get_auth_plugin("cop_dataspace").authenticate()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/data/geodata/sentinel/venv/lib/python3.13/site-packages/eodag/plugins/manager.py", line 272, in get_auth_plugin
    matching_url = getattr(associated_plugin.config, "api_endpoint", None)
                           ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'config'

This pull request addresses the issue in accordance with the EODAG documentation. Tested with EODAG version 3.10.0.

How to reproduce

i.sentinel.download settings=sentinel.txt limit=1 output=/data/tmp/