feat: inference passthrough provider (#1166)

##  What does this PR do?
In this PR, we implement a passthrough inference provider that works for
any endpoints that respect llama stack inference API definition.

## Test Plan
config some endpoint that respect llama stack inference API definition
and got the inference results successfully

<img width="1268" alt="Screenshot 2025-02-19 at 8 52 51 PM"
src="https://github.com/user-attachments/assets/447816e4-ea7a-4365-b90c-386dc7dcf4a1"
/>

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

@@ -0,0 +1,31 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from typing import Any, Dict, Optional
+
+from pydantic import BaseModel, Field, SecretStr
+
+from llama_stack.schema_utils import json_schema_type
+
+
+@json_schema_type
+class PassthroughImplConfig(BaseModel):
+    url: str = Field(
+        default=None,
+        description="The URL for the passthrough endpoint",
+    )
+
+    api_key: Optional[SecretStr] = Field(
+        default=None,
+        description="API Key for the passthrouth endpoint",
+    )
+
+    @classmethod
+    def sample_run_config(cls, **kwargs) -> Dict[str, Any]:
+        return {
+            "url": "${env.PASSTHROUGH_URL}",
+            "api_key": "${env.PASSTHROUGH_API_KEY}",
+        }