Introduce a "Router" layer for providers

Some providers need to be factorized and considered as thin routing
layers on top of other providers. Consider two examples:

- The inference API should be a routing layer over inference providers,
  routed using the "model" key
- The memory banks API is another instance where various memory bank
  types will be provided by independent providers (e.g., a vector store
  is served by Chroma while a keyvalue memory can be served by Redis or
  PGVector)

This commit introduces a generalized routing layer for this purpose.

llama_toolchain/core/package.py

@@ -4,22 +4,16 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
-import json
-import os
-from datetime import datetime
 from enum import Enum
 from typing import List, Optional
 
 import pkg_resources
-import yaml
-
-from llama_toolchain.common.config_dirs import BUILDS_BASE_DIR
-from llama_toolchain.common.exec import run_with_pty
-from llama_toolchain.common.serialize import EnumEncoder
 from pydantic import BaseModel
 
 from termcolor import cprint
 
+from llama_toolchain.common.exec import run_with_pty
+
 from llama_toolchain.core.datatypes import *  # noqa: F403
 from pathlib import Path
 
@@ -41,7 +35,7 @@ class ApiInput(BaseModel):
     provider: str
 
 
-def build_package(build_config: BuildConfig, build_file_path: Path):
+def build_image(build_config: BuildConfig, build_file_path: Path):
     package_deps = Dependencies(
         docker_image=build_config.distribution_spec.docker_image or "python:3.10-slim",
         pip_packages=SERVER_DEPENDENCIES,
@@ -49,17 +43,28 @@ def build_package(build_config: BuildConfig, build_file_path: Path):
 
     # extend package dependencies based on providers spec
     all_providers = api_providers()
-    for api_str, provider in build_config.distribution_spec.providers.items():
+    for (
+        api_str,
+        provider_or_providers,
+    ) in build_config.distribution_spec.providers.items():
         providers_for_api = all_providers[Api(api_str)]
-        if provider not in providers_for_api:
-            raise ValueError(
-                f"Provider `{provider}` is not available for API `{api_str}`"
-            )
 
-        provider_spec = providers_for_api[provider]
-        package_deps.pip_packages.extend(provider_spec.pip_packages)
-        if provider_spec.docker_image:
-            raise ValueError("A stack's dependencies cannot have a docker image")
+        providers = (
+            provider_or_providers
+            if isinstance(provider_or_providers, list)
+            else [provider_or_providers]
+        )
+
+        for provider in providers:
+            if provider not in providers_for_api:
+                raise ValueError(
+                    f"Provider `{provider}` is not available for API `{api_str}`"
+                )
+
+            provider_spec = providers_for_api[provider]
+            package_deps.pip_packages.extend(provider_spec.pip_packages)
+            if provider_spec.docker_image:
+                raise ValueError("A stack's dependencies cannot have a docker image")
 
     if build_config.image_type == ImageType.docker.value:
         script = pkg_resources.resource_filename(