feat: add ModelScope backend support with modelscope:// protocol

[pmady]

What this PR does / why we need it:

Adds a new backend for downloading models and datasets from ModelScope Hub using the modelscope:// URL scheme, similar to the existing Hugging Face backend.

URL Format

modelscope://[<repo_type>/]<owner>/<repo>[/<path>][@<revision>]

Examples

# Download a model repository
dfget modelscope://inclusionAI/Ling-1T -O /tmp/ling-1t/ --recursive

# Download a single file
dfget modelscope://inclusionAI/Ling-1T/config.json -O /tmp/config.json

# Download with authentication
dfget modelscope://inclusionAI/Ling-1T/config.json -O /tmp/config.json --ms-token=<token>

# Download a dataset
dfget modelscope://datasets/damo/squad-zh/train.json -O /tmp/train.json

# Download from a specific revision
dfget modelscope://inclusionAI/Ling-1T/config.json@v1.0 -O /tmp/config.json

Features

• Support for models and datasets repository types
• File listing via ModelScope API (/api/v1/models/{id}/repo/files) for recursive downloads
• File download via ModelScope API (/api/v1/models/{id}/repo?FilePath=...)
• Authentication via --ms-token CLI flag (injected as Authorization: Bearer header)
• Range download support
• Default revision is master (matching ModelScope convention)

Which issue(s) this PR fixes:

Closes dragonflyoss/dragonfly#4420

Does this PR introduce a user-facing change?

Yes. Users can now download files and repositories from ModelScope Hub using dfget modelscope://... with P2P acceleration.

[pmady]

Hi maintainers, could you please add the enhancement label to this PR? The PR Label check requires one of: bug, enhancement, documentation, or dependencies. Thank you!

[codecov]

Codecov Report

❌ Patch coverage is 52.52101% with 226 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.52%. Comparing base (035ae91) to head (9597b39).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client-backend/src/modelscope.rs	51.20%	222 Missing ⚠️
dragonfly-client/src/bin/dfget/main.rs	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1673      +/-   ##
==========================================
- Coverage   50.83%   50.52%   -0.32%     
==========================================
  Files          83       84       +1     
  Lines       20029    20631     +602     
==========================================
+ Hits        10182    10424     +242     
- Misses       9847    10207     +360     

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/lib.rs	96.24% <100.00%> (+0.19%)	⬆️
dragonfly-client/src/bin/dfget/main.rs	49.92% <33.33%> (-0.08%)	⬇️
dragonfly-client-backend/src/modelscope.rs	51.20% <51.20%> (ø)

... and 4 files with indirect coverage changes

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[gaius-qi]

@pmady Thanks, I'll finish the review by this week.