Feat openml models and auth

[mardalla]

Summary:

• Skip authorization_test call to avoid JSONDecodeError from the aiod SDK.
• Add support for indexing OpenML flows as ml_models assets (via new --asset-type model and associated code paths).
• Keep existing dataset behaviour intact (--asset-type dataset).

Notes:

• Tested locally with --mode id, --mode since, and --mode all for datasets.
• Basic sanity check for models with a few flow IDs; indexing completes and assets appear in AIoD.