feat(auth, rfc): Add support for Bearer (api_key) Authentication (#1626)

This PR adds support (or is a proposal for) for supporting API KEY
authentication on the Llama Stack server end. `llama-stack-client`
already supports accepting an api_key parameter and passes it down
through every request as an `Authentication: ` header.

Currently, Llama Stack does not propose APIs for handling authentication
or authorization for resources of any kind. Given that, and the fact
that any deployment will typically have _some_ authentication system
present, we simply adopt a delegation mechanism: delegate to an HTTPS
endpoint performing key management / authentication.

It is configured via: 
```yaml
server: 
   auth:
     endpoint: <...>
```

in the run.yaml configuration.


## How It Works

When authentication is enabled:

1. Every API request must include an `Authorization: Bearer <token>`
header
2. The server will send a _POST_ validation request to the configured
endpoint with the following payload:
   ```json
   {
     "api_key": "<token>",
     "request": {
       "path": "/api/path",
       "headers": { "header1": "value1", ... },
       "params": { "param1": "value1", ... }
     }
   }
   ```
3. If the authentication endpoint returns a 200 status code, the request
is allowed to proceed
4. If the authentication endpoint returns any other status code, a 401
Unauthorized response is returned

## Test Plan

Unit tests

tests/unit/server/test_auth.py

@@ -0,0 +1,124 @@
+# 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 unittest.mock import AsyncMock, patch
+
+import pytest
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+from llama_stack.distribution.server.auth import AuthenticationMiddleware
+
+
+@pytest.fixture
+def mock_auth_endpoint():
+    return "http://mock-auth-service/validate"
+
+
+@pytest.fixture
+def valid_api_key():
+    return "valid_api_key_12345"
+
+
+@pytest.fixture
+def invalid_api_key():
+    return "invalid_api_key_67890"
+
+
+@pytest.fixture
+def app(mock_auth_endpoint):
+    app = FastAPI()
+    app.add_middleware(AuthenticationMiddleware, auth_endpoint=mock_auth_endpoint)
+
+    @app.get("/test")
+    def test_endpoint():
+        return {"message": "Authentication successful"}
+
+    return app
+
+
+@pytest.fixture
+def client(app):
+    return TestClient(app)
+
+
+async def mock_post_success(*args, **kwargs):
+    mock_response = AsyncMock()
+    mock_response.status_code = 200
+    return mock_response
+
+
+async def mock_post_failure(*args, **kwargs):
+    mock_response = AsyncMock()
+    mock_response.status_code = 401
+    return mock_response
+
+
+async def mock_post_exception(*args, **kwargs):
+    raise Exception("Connection error")
+
+
+def test_missing_auth_header(client):
+    response = client.get("/test")
+    assert response.status_code == 401
+    assert "Missing or invalid Authorization header" in response.json()["error"]["message"]
+
+
+def test_invalid_auth_header_format(client):
+    response = client.get("/test", headers={"Authorization": "InvalidFormat token123"})
+    assert response.status_code == 401
+    assert "Missing or invalid Authorization header" in response.json()["error"]["message"]
+
+
+@patch("httpx.AsyncClient.post", new=mock_post_success)
+def test_valid_authentication(client, valid_api_key):
+    response = client.get("/test", headers={"Authorization": f"Bearer {valid_api_key}"})
+    assert response.status_code == 200
+    assert response.json() == {"message": "Authentication successful"}
+
+
+@patch("httpx.AsyncClient.post", new=mock_post_failure)
+def test_invalid_authentication(client, invalid_api_key):
+    response = client.get("/test", headers={"Authorization": f"Bearer {invalid_api_key}"})
+    assert response.status_code == 401
+    assert "Authentication failed" in response.json()["error"]["message"]
+
+
+@patch("httpx.AsyncClient.post", new=mock_post_exception)
+def test_auth_service_error(client, valid_api_key):
+    response = client.get("/test", headers={"Authorization": f"Bearer {valid_api_key}"})
+    assert response.status_code == 401
+    assert "Authentication service error" in response.json()["error"]["message"]
+
+
+def test_auth_request_payload(client, valid_api_key, mock_auth_endpoint):
+    with patch("httpx.AsyncClient.post") as mock_post:
+        mock_response = AsyncMock()
+        mock_response.status_code = 200
+        mock_post.return_value = mock_response
+
+        client.get(
+            "/test?param1=value1&param2=value2",
+            headers={
+                "Authorization": f"Bearer {valid_api_key}",
+                "User-Agent": "TestClient",
+                "Content-Type": "application/json",
+            },
+        )
+
+        # Check that the auth endpoint was called with the correct payload
+        call_args = mock_post.call_args
+        assert call_args is not None
+
+        url, kwargs = call_args[0][0], call_args[1]
+        assert url == mock_auth_endpoint
+
+        payload = kwargs["json"]
+        assert payload["api_key"] == valid_api_key
+        assert payload["request"]["path"] == "/test"
+        assert "authorization" in payload["request"]["headers"]
+        assert "param1" in payload["request"]["params"]
+        assert "param2" in payload["request"]["params"]