Add HuggingFace auth routing (`HuggingFaceAuth`)

[Kamilbenkirane]

Sub-issue of #148

Implement auth=HuggingFaceAuth() to reroute compatible providers through HuggingFace's router (router.huggingface.co) using HF_TOKEN — exactly like auth=GoogleADC() reroutes providers through Vertex AI today.

This enables providers like Together, nscale, fal-ai, Replicate etc. to be accessed via HF billing, without needing separate API keys per provider.

Assumes target providers already exist as their own Provider.* entries. This issue covers the auth routing layer only.

See #166 for architecture decision and URL rewriting details.

Done when: a user can pass auth=HuggingFaceAuth() to a supported provider and requests are transparently routed through router.huggingface.co with HF_TOKEN as the credential.

@LeoLejeune