[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.

docs/openapi_generator/strong_typing/mapping.py

@@ -0,0 +1,42 @@
+# 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.
+
+"""
+Type-safe data interchange for Python data classes.
+
+:see: https://github.com/hunyadi/strong_typing
+"""
+
+import keyword
+from typing import Optional
+
+from .auxiliary import Alias
+from .inspection import get_annotation
+
+
+def python_field_to_json_property(
+    python_id: str, python_type: Optional[object] = None
+) -> str:
+    """
+    Map a Python field identifier to a JSON property name.
+
+    Authors may use an underscore appended at the end of a Python identifier as per PEP 8 if it clashes with a Python
+    keyword: e.g. `in` would become `in_` and `from` would become `from_`. Remove these suffixes when exporting to JSON.
+
+    Authors may supply an explicit alias with the type annotation `Alias`, e.g. `Annotated[MyType, Alias("alias")]`.
+    """
+
+    if python_type is not None:
+        alias = get_annotation(python_type, Alias)
+        if alias:
+            return alias.name
+
+    if python_id.endswith("_"):
+        id = python_id[:-1]
+        if keyword.iskeyword(id):
+            return id
+
+    return python_id