[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/distribution/routers/routing_tables.py

@@ -0,0 +1,116 @@
+# 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 typing import Any, List, Optional, Tuple
+
+from llama_models.sku_list import resolve_model
+from llama_models.llama3.api.datatypes import *  # noqa: F403
+
+from llama_stack.apis.models import *  # noqa: F403
+from llama_stack.apis.shields import *  # noqa: F403
+from llama_stack.apis.memory_banks import *  # noqa: F403
+
+from llama_stack.distribution.datatypes import *  # noqa: F403
+
+
+class CommonRoutingTableImpl(RoutingTable):
+    def __init__(
+        self,
+        inner_impls: List[Tuple[str, Any]],
+        routing_table_config: Dict[str, List[RoutableProviderConfig]],
+    ) -> None:
+        self.providers = {k: v for k, v in inner_impls}
+        self.routing_keys = list(self.providers.keys())
+        self.routing_table_config = routing_table_config
+
+    async def initialize(self) -> None:
+        pass
+
+    async def shutdown(self) -> None:
+        for p in self.providers.values():
+            await p.shutdown()
+
+    def get_provider_impl(self, routing_key: str) -> Optional[Any]:
+        return self.providers.get(routing_key)
+
+    def get_routing_keys(self) -> List[str]:
+        return self.routing_keys
+
+    def get_provider_config(self, routing_key: str) -> Optional[GenericProviderConfig]:
+        for entry in self.routing_table_config:
+            if entry.routing_key == routing_key:
+                return entry
+        return None
+
+
+class ModelsRoutingTable(CommonRoutingTableImpl, Models):
+
+    async def list_models(self) -> List[ModelServingSpec]:
+        specs = []
+        for entry in self.routing_table_config:
+            model_id = entry.routing_key
+            specs.append(
+                ModelServingSpec(
+                    llama_model=resolve_model(model_id),
+                    provider_config=entry,
+                )
+            )
+        return specs
+
+    async def get_model(self, core_model_id: str) -> Optional[ModelServingSpec]:
+        for entry in self.routing_table_config:
+            if entry.routing_key == core_model_id:
+                return ModelServingSpec(
+                    llama_model=resolve_model(core_model_id),
+                    provider_config=entry,
+                )
+        return None
+
+
+class ShieldsRoutingTable(CommonRoutingTableImpl, Shields):
+
+    async def list_shields(self) -> List[ShieldSpec]:
+        specs = []
+        for entry in self.routing_table_config:
+            specs.append(
+                ShieldSpec(
+                    shield_type=entry.routing_key,
+                    provider_config=entry,
+                )
+            )
+        return specs
+
+    async def get_shield(self, shield_type: str) -> Optional[ShieldSpec]:
+        for entry in self.routing_table_config:
+            if entry.routing_key == shield_type:
+                return ShieldSpec(
+                    shield_type=entry.routing_key,
+                    provider_config=entry,
+                )
+        return None
+
+
+class MemoryBanksRoutingTable(CommonRoutingTableImpl, MemoryBanks):
+
+    async def list_available_memory_banks(self) -> List[MemoryBankSpec]:
+        specs = []
+        for entry in self.routing_table_config:
+            specs.append(
+                MemoryBankSpec(
+                    bank_type=entry.routing_key,
+                    provider_config=entry,
+                )
+            )
+        return specs
+
+    async def get_serving_memory_bank(self, bank_type: str) -> Optional[MemoryBankSpec]:
+        for entry in self.routing_table_config:
+            if entry.routing_key == bank_type:
+                return MemoryBankSpec(
+                    bank_type=entry.routing_key,
+                    provider_config=entry,
+                )
+        return None