[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/utils/kvstore/redis/redis.py

@@ -0,0 +1,52 @@
+# 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 datetime import datetime
+from typing import List, Optional
+
+from redis.asyncio import Redis
+
+from ..api import *  # noqa: F403
+from ..config import RedisKVStoreConfig
+
+
+class RedisKVStoreImpl(KVStore):
+    def __init__(self, config: RedisKVStoreConfig):
+        self.config = config
+
+    async def initialize(self) -> None:
+        self.redis = Redis.from_url(self.config.url)
+
+    def _namespaced_key(self, key: str) -> str:
+        if not self.config.namespace:
+            return key
+        return f"{self.config.namespace}:{key}"
+
+    async def set(
+        self, key: str, value: str, expiration: Optional[datetime] = None
+    ) -> None:
+        key = self._namespaced_key(key)
+        await self.redis.set(key, value)
+        if expiration:
+            await self.redis.expireat(key, expiration)
+
+    async def get(self, key: str) -> Optional[str]:
+        key = self._namespaced_key(key)
+        value = await self.redis.get(key)
+        if value is None:
+            return None
+        ttl = await self.redis.ttl(key)
+        return value
+
+    async def delete(self, key: str) -> None:
+        key = self._namespaced_key(key)
+        await self.redis.delete(key)
+
+    async def range(self, start_key: str, end_key: str) -> List[str]:
+        start_key = self._namespaced_key(start_key)
+        end_key = self._namespaced_key(end_key)
+
+        return await self.redis.zrangebylex(start_key, end_key)