Fix: MotherDuck token from plugin config should override env var

[guenp]

This PR fixes a bug:

• When passing a token via the config, this didn't work because there is a typo (by yours truly 😬) here.

The PR also addresses how a user can pass the MotherDuck token to duckdb.connect:

• Adds a motherduck_token property to DuckDBCredentials that fetches it from the plugin config.
• The user can specify the token in the path (e.g. "md:?motherduck_token=<token>". This will override a motherduck_token environment variable if it exists.
• We no longer use an explicit SET command and instead pass a config dictionary to duckdb.connect. Any config options passed this way will override the corresponding environment variable as well as any options set in the path.

So the behavior is now the same as in duckdb: env var < path < config. (where left < right means "right overrides left").

[guenp]

All checks are green: https://github.com/motherduckdb/dbt-duckdb/actions/runs/7909652885/job/21591671925

After we merge this we should be OK to release! 🎉