Add extensible `DatabaseManager` class enabling custom database backends support

[andrii-i]

References

• Enables https://github.com/jupyter-ai-contrib/jupyter-scheduler-k8s/pull/5

Problem

Currently, while users can customize the database URL via db_url traitlet, the DatabaseManager.create_session and DatabaseManager.create_tables functions in orm.py are tightly coupled to SQLAlchemy, which essentially means that while users can specify different database URLs they must be SQLAlchemy-compatible which is limiting.

Proposed solution / Code changes

Introduce extensible DatabaseManager class that can be configured via traitlets.

User-facing changes

None

Backwards-incompatible changes

None