Add offline SQLite backend

[cfuselli]

This pull request introduces offline (SQLite-based) backend support to utilix, allowing core database and GridFS operations to work seamlessly when disconnected from the main MongoDB service. The main changes involve integrating the new sqlite_backend and updating the relevant classes and methods to use it when configured.

Offline backend integration:

• Added imports for sqlite_backend and its key classes/functions (OfflineGridFS, OfflineSQLiteCollection, SQLiteConfig, _load_sqlite_config) in utilix/__init__.py, utilix/mongo_storage.py, and utilix/rundb.py to enable offline support throughout the package. [1] [2] [3]

GridFS (file storage) offline support:

• Updated the GridFsBase class in utilix/mongo_storage.py to initialize and use OfflineGridFS when offline mode is detected, including offline-safe overrides for file listing, config existence checks, and file downloads. This ensures file storage and retrieval continue to work in offline scenarios. [1] [2]

RunDB (database) offline support:

• Modified utilix/rundb.py to provide an _sqlite_collection function and update xent_collection to use the offline SQLite backend if enabled, allowing database queries to fallback to the local SQLite database when offline.

General improvements and error handling:

• Added robust error handling for configuration loading and type checking for cache directories, improving reliability and user feedback when initializing offline backends.

Consistency and code reuse:

• Ensured consistent use of the offline backend throughout the codebase, with shared logic for configuration loading and backend selection in both file and database operations. [1] [2] [3]