Updates to ScPCAr for ScPCA API changes

[jashapiro]

As the new version of the ScPCA portal API comes online, we will need to update this package to support new features and ensure compatibility.

This Epic tracks the steps involved in that process.

The general steps will include:

• Describing any changes that affect the current implementation, filing issues here to track those
• Describing new functions we want to add in new issues
• Completing those issues!

[jashapiro]

Based on testing, the current client continues to work unmodified with the updated API currently in staging (as of Dec 17, 2025).

We may want to update the client to use the ccdl-datasets endpoint to get information for downloading precomputed datasets, but this seems at least for now to be optional.