Add dynamic authentication token forwarding support for vLLM provider

This enables per-request authentication tokens for vLLM providers, supporting use cases like RAG operations where different requests may need different authentication tokens. The implementation follows the same pattern as other providers like Together AI, Fireworks, and Passthrough.

- Add LiteLLMOpenAIMixin that manages the vllm_api_token properly

Usage:

- Static: VLLM_API_TOKEN env var or config.api_token
- Dynamic: X-LlamaStack-Provider-Data header with vllm_api_token
All existing functionality is preserved while adding new dynamic capabilities.

Signed-off-by: Akram Ben Aissi <akram.benaissi@gmail.com>

llama_stack/providers/registry/inference.py

@@ -78,6 +78,7 @@ def available_providers() -> list[ProviderSpec]:
                 pip_packages=[],
                 module="llama_stack.providers.remote.inference.vllm",
                 config_class="llama_stack.providers.remote.inference.vllm.VLLMInferenceAdapterConfig",
+                provider_data_validator="llama_stack.providers.remote.inference.vllm.VLLMProviderDataValidator",
                 description="Remote vLLM inference provider for connecting to vLLM servers.",
             ),
         ),