Easy OAuth with providers other than Hugging Face

OAuth is easy when using Hugging Face as the provider, but setting it up with other providers (Google, GitHub, generic OIDC, etc.) is painful right now.

Proposal

• Built-in support for common OAuth/OIDC providers with the same simple API as HF OAuth (e.g. gr.LoginButton(provider=...))
• Works for Blocks apps and gr.Server apps
• Consistent with the MCP + OAuth work in #11472

Related: #8592 (custom login page), #11472 (HF OAuth with MCP)