Add support for DuckDB

[RasmusSkytte]

Intent

This PR adds support for DuckDB.

Approach

Several of the S3 functions interfacing directly with the backend needed an extension for DuckDB.

In addition, get_connection() was converted to a S3 generic to better separate the different arguments needed for each backend.

Known issues

The DuckDB backend is showing a lot warning messages about garbage collection.

Database is garbage-collected, use dbDisconnect(con, shutdown=TRUE) or duckdb::duckdb_shutdown(drv) to avoid this.

Looking into the root cause of this, we are using shutdown = TRUE..

Turns out we needed to shutdown the driver AND the connection.

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[LasseEngboChr]

Just two small items to look at - and great that it was relatively easy to add a fourth backend :-)