Provide option to open and close connection for every cell

[mettekou]

@edublancas Aside from the default option of keeping an aliased database connection open as long as the Jupyter kernel process runs, provide a configuration option to automatically open the connection before every cell runs and close it after it has run. This is particularly useful when using DuckDB and connecting and writing to the same database from different Jupyter notebooks and therefore kernel processes at the same time, e.g. when writing a report using Jupyter Book.