Add support for parallel workers for folder downloads.

[GustaMantovani]

Introduces a workers parameter to both the download_folder function and the CLI, allowing users to specify an integer number of workers or "auto" (which uses os.cpu_count()).

Under the hood, a helper function spawns a configurable number of threads and stores them in a pool. These threads consume from a shared input queue of (file_id, destination_path) tuples, pre-populated by _get_directory_structure().

If skip_download=True, no workers are created. By default, workers=1. When more than one worker is specified, the concurrent threads cause the tqdm progress display to misbehave, so we temporarily enforce quiet=True; this will be addressed in an upcoming PR.

In my tests, parallel downloads substantially increased throughput for folders containing many files.