feat: Added support for Flask-SQLAlchemy-Lite session management

[mstewart1409]

This pull request adds support for Flask-SQLAlchemy-Lite as a session backend, allowing users to use flask_sqlalchemy_lite for session storage in addition to the existing SQLAlchemy options. The implementation includes a new session interface, configuration options, documentation updates, and tests.

New Flask-SQLAlchemy-Lite session backend:

• Added a new SqlAlchemyLiteSessionInterface and related session model logic in src/flask_session/sqlalchemy_lite/sqlalchemy_lite.py, enabling session storage via flask_sqlalchemy_lite.
• Integrated the new backend into the session interface selection logic in src/flask_session/__init__.py, including support for configuration parameters such as SESSION_SQLALCHEMY_BASE_MODEL. [1] [2]
• Registered the new session interface in the sqlalchemy_lite module's __init__.py.
• Added a default value for SESSION_SQLALCHEMY_BASE_MODEL in src/flask_session/defaults.py.

Documentation and configuration:

• Updated documentation in docs/config_reference.rst and docs/installation.rst to describe the new backend, its configuration parameters, and installation instructions. [1] [2] [3]
• Added intersphinx mapping for flask-sqlalchemy-lite in docs/conf.py.
• Specified the Sphinx configuration file in .readthedocs.yaml.

Dependency management:

• Added flask-sqlalchemy-lite to the development, documentation, and testing dependencies in requirements/docs.in, requirements/docs.txt, requirements/dev.txt, pyproject.toml, and updated sqlalchemy version. [1] [2] [3] [4] [5]

Testing:

• Added a new test suite in tests/test_sqlalchemy_lite.py to verify correct operation of the new session backend, including session storage and retrieval.