DuckDB: Add support for installing and loading DuckDB extensions

[ramalingamt]

The DuckDB driver currently loads only the httpfs extension by default. There is no way to add additional extensions to the runtime like the iceberg or delta extensions.

Propose to add a custom environment variable (say CUBEJS_DB_DUCKDB_EXTENSIONS) with a list of extensions to load. This was suggested by @igorlukanin in Slack as a possible solution.

Slack Message