Add file attachments to units via Python SDK

You can now attach files directly to units using the Python SDK v2.1.0. Upload photos, test reports, calibration data, or any file and link it to a unit in two lines of code.

• Added attachments.upload() to handle file uploads in a single call
• Added attachments.download() to download attachments from units or runs
• Added attachments support to the Update Unit endpoint
• Renamed the attachment finalization endpoint for clarity