[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/adapters/inference/fireworks/fireworks.py

@@ -6,14 +6,14 @@
 
 from typing import AsyncGenerator
 
+from fireworks.client import Fireworks
+
 from llama_models.llama3.api.chat_format import ChatFormat
 
 from llama_models.llama3.api.datatypes import Message, StopReason
 from llama_models.llama3.api.tokenizer import Tokenizer
 from llama_models.sku_list import resolve_model
 
-from fireworks.client import Fireworks
-
 from llama_stack.apis.inference import *  # noqa: F403
 from llama_stack.providers.utils.inference.prepare_messages import prepare_messages
 
@@ -42,7 +42,14 @@ class FireworksInferenceAdapter(Inference):
     async def shutdown(self) -> None:
         pass
 
-    async def completion(self, request: CompletionRequest) -> AsyncGenerator:
+    async def completion(
+        self,
+        model: str,
+        content: InterleavedTextMedia,
+        sampling_params: Optional[SamplingParams] = SamplingParams(),
+        stream: Optional[bool] = False,
+        logprobs: Optional[LogProbConfig] = None,
+    ) -> AsyncGenerator:
         raise NotImplementedError()
 
     def _messages_to_fireworks_messages(self, messages: list[Message]) -> list: