Store firebase data into subcollections inside session

[mario-bermonti]

Description

Device and session metadata are now stored into separate docs in dedicated subcollections.

Thes changes improve code maintainability, protect data integrity (e.g., being overriden), and make it easier to convert back into data models used by cognitive_tasks.

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🐞 Bug fix (non-breaking change that fixes an issue)
• 🔧 Maintenance (non-breaking change that improves code)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix
• 🔐 Improvements to the CI workflow

Checklist

• I've read the project's code of conduct.
• I've read the contributing guide.
• I've written tests for all new methods and classes that I created.