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/common/prompt_for_config.py

@@ -27,6 +27,12 @@ def is_list_of_primitives(field_type):
     return False
 
 
+def is_basemodel_without_fields(typ):
+    return (
+        inspect.isclass(typ) and issubclass(typ, BaseModel) and len(typ.__fields__) == 0
+    )
+
+
 def can_recurse(typ):
     return (
         inspect.isclass(typ) and issubclass(typ, BaseModel) and len(typ.__fields__) > 0
@@ -151,6 +157,11 @@ def prompt_for_config(
         if get_origin(field_type) is Literal:
             continue
 
+        # Skip fields with no type annotations
+        if is_basemodel_without_fields(field_type):
+            config_data[field_name] = field_type()
+            continue
+
         if inspect.isclass(field_type) and issubclass(field_type, Enum):
             prompt = f"Choose {field_name} (options: {', '.join(e.name for e in field_type)}):"
             while True:
@@ -254,6 +265,20 @@ def prompt_for_config(
                                 print(f"{str(e)}")
                                 continue
 
+                        elif get_origin(field_type) is dict:
+                            try:
+                                value = json.loads(user_input)
+                                if not isinstance(value, dict):
+                                    raise ValueError(
+                                        "Input must be a JSON-encoded dictionary"
+                                    )
+
+                            except json.JSONDecodeError:
+                                print(
+                                    "Invalid JSON. Please enter a valid JSON-encoded dict."
+                                )
+                                continue
+
                         # Convert the input to the correct type
                         elif inspect.isclass(field_type) and issubclass(
                             field_type, BaseModel