feat: change download() and download_dir() to conditionally initialize backend-specific configs

[gaius-qi]

Description

Refactor backend scheme constants:

• Rename HDFS_SCHEME to SCHEME in hdfs.rs
• Rename HUGGING_FACE_SCHEME to SCHEME in hugging_face.rs
• Rename MODEL_SCOPE_SCHEME to SCHEME in model_scope.rs

Add Scheme::is_supported() helper method to object_storage.rs to check if a given URL scheme is a supported object storage protocol, along with corresponding unit tests.

Refactor download() and download_dir() in dfget/main.rs to conditionally initialize backend-specific configs (object_storage, hdfs, hugging_face, model_scope) based on the URL scheme instead of always populating all configs unconditionally.

Related Issue

Motivation and Context

Screenshots (if appropriate)

[BraveY]

LGTM

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 63 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.29%. Comparing base (0322a7d) to head (360ebce).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client/src/bin/dfget/main.rs	0.00%	62 Missing ⚠️
dragonfly-client-backend/src/hugging_face.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1713      +/-   ##
==========================================
- Coverage   50.29%   50.29%   -0.01%     
==========================================
  Files          86       86              
  Lines       21963    21977      +14     
==========================================
+ Hits        11046    11053       +7     
- Misses      10917    10924       +7     

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/hdfs.rs	40.90% <100.00%> (ø)
dragonfly-client-backend/src/lib.rs	96.30% <100.00%> (ø)
dragonfly-client-backend/src/model_scope.rs	42.72% <100.00%> (-0.26%)	⬇️
dragonfly-client-backend/src/object_storage.rs	88.77% <100.00%> (+0.14%)	⬆️
dragonfly-client-backend/src/hugging_face.rs	35.49% <75.00%> (-0.32%)	⬇️
dragonfly-client/src/bin/dfget/main.rs	49.33% <0.00%> (-0.26%)	⬇️

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