Support recursive repository file discovery and options for local repositories

[dkuku]

This PR adds support for recursive file discovery when loading repository files from HuggingFace Hub and local directories, and brings feature parity to {:local, directory, options}.

Changes

1. HuggingFace Hub Tree Listing (HuggingFace.Hub.file_listing_url/4):

   • Added support for :recursive option, defaulting to true (passing ?recursive=true to the Hugging Face tree API).
   • Allows discovering and downloading files located within nested module or component directories (e.g. 1_Pooling/config.json, 2_Dense/model.safetensors).
   • Backwards compatible with existing 3-argument callers while allowing explicit opt-out with recursive: false.

2. Local Directory Loading ({:local, dir} and {:local, dir, opts}):

   • get_repo_files searches up to depth 2 (dir/* and dir/*/*) when recursive: true (the default), discovering files inside module subdirectories while avoiding unbounded filesystem traversal.
   • Added support for {:local, directory, options} with :subdir and :recursive options, matching {:hf, repository_id, options}.
   • Updated download/3 and normalize_repository!/1 accordingly.

3. Tests:

   • Added unit tests in HubTest for recursive and non-recursive file_listing_url.
   • Added unit tests in BumblebeeTest for local directory loading with nested subdirectories, :subdir, and depth-2 boundary enforcement.