chore: add provider-data-api-key support to openaimixin (#3639)

# What does this PR do?

the LiteLLMOpenAIMixin provides support for reading key from provider
data (headers users send).

this adds the same functionality to the OpenAIMixin.

this is infrastructure for migrating providers.


## Test Plan

ci w/ new tests

llama_stack/providers/utils/inference/model_registry.py

@@ -63,7 +63,7 @@ class ModelRegistryHelper(ModelsProtocolPrivate):
         model_entries: list[ProviderModelEntry] | None = None,
         allowed_models: list[str] | None = None,
     ):
-        self.allowed_models = allowed_models
+        self.allowed_models = allowed_models if allowed_models else []
 
         self.alias_to_provider_id_map = {}
         self.provider_id_to_llama_model_map = {}