[API Updates] Model / shield / memory-bank routing + agent persistence + support for private headers (#92)

This is yet another of those large PRs (hopefully we will have less and less of them as things mature fast). This one introduces substantial improvements and some simplifications to the stack.

Most important bits:

* Agents reference implementation now has support for session / turn persistence. The default implementation uses sqlite but there's also support for using Redis.

* We have re-architected the structure of the Stack APIs to allow for more flexible routing. The motivating use cases are:
  - routing model A to ollama and model B to a remote provider like Together
  - routing shield A to local impl while shield B to a remote provider like Bedrock
  - routing a vector memory bank to Weaviate while routing a keyvalue memory bank to Redis

* Support for provider specific parameters to be passed from the clients. A client can pass data using `x_llamastack_provider_data` parameter which can be type-checked and provided to the Adapter implementations.

llama_stack/providers/impls/meta_reference/agents/tests/test_chat_agent.py

@@ -5,7 +5,6 @@
 # the root directory of this source tree.
 
 from typing import AsyncIterator, List, Optional, Union
-from unittest.mock import MagicMock
 
 import pytest
 
@@ -79,10 +78,10 @@ class MockInferenceAPI:
 
 
 class MockSafetyAPI:
-    async def run_shields(
-        self, messages: List[Message], shields: List[MagicMock]
-    ) -> List[ShieldResponse]:
-        return [ShieldResponse(shield_type="mock_shield", is_violation=False)]
+    async def run_shield(
+        self, shield_type: str, messages: List[Message]
+    ) -> RunShieldResponse:
+        return RunShieldResponse(violation=None)
 
 
 class MockMemoryAPI:
@@ -185,6 +184,7 @@ async def chat_agent(mock_inference_api, mock_safety_api, mock_memory_api):
             # ),
         ],
         tool_choice=ToolChoice.auto,
+        enable_session_persistence=False,
         input_shields=[],
         output_shields=[],
     )
@@ -221,13 +221,13 @@ async def test_chat_agent_create_and_execute_turn(chat_agent):
 
 
 @pytest.mark.asyncio
-async def test_run_shields_wrapper(chat_agent):
+async def test_run_multiple_shields_wrapper(chat_agent):
     messages = [UserMessage(content="Test message")]
-    shields = [ShieldDefinition(shield_type="test_shield")]
+    shields = ["test_shield"]
 
     responses = [
         chunk
-        async for chunk in chat_agent.run_shields_wrapper(
+        async for chunk in chat_agent.run_multiple_shields_wrapper(
             turn_id="test_turn_id",
             messages=messages,
             shields=shields,