Use official Cloud Firestore package

[yoshiask]

Currently, FluentStoreApi talked to the Firebase API by manually constructing REST API requests. This was done because the official .NET package doesn't support non-desktop paradigms. This is no longer an issue since Fluent Store uses WinUI 3.

Planned steps:

• [ ] Abstract away usage of the homegrown implementation
• [ ] Implement the abstraction using the official library
• [ ] Add UAP10 as a separate target framework
• [ ] Use the appropriate implementation for the target

[yoshiask]

This may resolve #70.

[yoshiask]

Upon further investigation, the Cloud Firestore libraries are an absolute mess. Instead, we'll stick with the current implementation and favor moving to a completely different platform (maybe Supabase) in the future.