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/__init__.py

@@ -0,0 +1,23 @@
+# 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 pydantic import BaseModel
+
+from .config import PassthroughImplConfig
+
+
+class PassthroughProviderDataValidator(BaseModel):
+    url: str
+    api_key: str
+
+
+async def get_adapter_impl(config: PassthroughImplConfig, _deps):
+    from .passthrough import PassthroughInferenceAdapter
+
+    assert isinstance(config, PassthroughImplConfig), f"Unexpected config type: {type(config)}"
+    impl = PassthroughInferenceAdapter(config)
+    await impl.initialize()
+    return impl