feat: use SecretStr for inference provider auth credentials (#3724)

# What does this PR do?

use SecretStr for OpenAIMixin providers

- RemoteInferenceProviderConfig now has auth_credential: SecretStr
- the default alias is api_key (most common name)
- some providers override to use api_token (RunPod, vLLM, Databricks)
- some providers exclude it (Ollama, TGI, Vertex AI)

addresses #3517 

## Test Plan

ci w/ new tests

llama_stack/providers/remote/inference/together/config.py

@@ -6,7 +6,7 @@
 
 from typing import Any
 
-from pydantic import Field, SecretStr
+from pydantic import Field
 
 from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
 from llama_stack.schema_utils import json_schema_type
@@ -18,10 +18,6 @@ class TogetherImplConfig(RemoteInferenceProviderConfig):
         default="https://api.together.xyz/v1",
         description="The URL for the Together AI server",
     )
-    api_key: SecretStr | None = Field(
-        default=None,
-        description="The Together AI API Key",
-    )
 
     @classmethod
     def sample_run_config(cls, **kwargs) -> dict[str, Any]:

llama_stack/providers/remote/inference/together/together.py

@@ -39,15 +39,12 @@ class TogetherInferenceAdapter(OpenAIMixin, NeedsRequestProviderData):
 
     provider_data_api_key_field: str = "together_api_key"
 
-    def get_api_key(self):
-        return self.config.api_key.get_secret_value() if self.config.api_key else None
-
     def get_base_url(self):
         return BASE_URL
 
     def _get_client(self) -> AsyncTogether:
         together_api_key = None
-        config_api_key = self.config.api_key.get_secret_value() if self.config.api_key else None
+        config_api_key = self.config.auth_credential.get_secret_value() if self.config.auth_credential else None
         if config_api_key:
             together_api_key = config_api_key
         else: