refactor(hf): use the official hf scheme instead of huggingface

[kszucs]

Which issue does this PR close?

Closes #.

Rationale for this change

The official Hugging Face scheme is hf://. Aligning OpenDAL's naming with the official convention makes the API more intuitive and consistent with the broader Hugging Face ecosystem.

What changes are included in this PR?

• Renamed all code constructs from Huggingface* to Hf* (e.g. HuggingfaceBuilder -> HfBuilder, HuggingfaceConfig -> HfConfig, etc.)
• Moved the services/huggingface directory to services/hf and renamed the crate from opendal-service-huggingface to opendal-service-hf
• Registered both hf:// (primary) and huggingface:// (alias) URI schemes
• Introduced services-hf feature flag as the primary, with services-huggingface kept as a backward-compatible alias (services-huggingface = ["services-hf"])
• Updated Python, Java, and Node.js bindings to use the new naming convention
• Updated CI workflow to use the services-hf feature flag

Are there any user-facing changes?

Yes. The public API types are renamed:

• Huggingface -> Hf
• HuggingfaceConfig -> HfConfig
• Feature flag services-hf is now primary (old services-huggingface still works as alias)
• Python Scheme.Huggingface -> Scheme.Hf
• Both hf:// and huggingface:// URI schemes are supported

AI Usage Statement

This PR was authored with the assistance of Claude Code.

[tisonkun]

Thanks for your contribution! LGTM.

cc @Xuanwo if you'd take a look.

[kszucs]

cc @lhoestq

[tisonkun]

The Go CI failure seems to be unrelated - need to bump toolchain version separately.

[tisonkun]

@Xuanwo you may track an item to add a changelog entry.

[kszucs]

Thank you!