refactor(storage): make { kvstore, sqlstore } as llama stack "internal" APIs (#4181)

These primitives (used both by the Stack as well as provider
implementations) can be thought of fruitfully as internal-only APIs
which can themselves have multiple implementations. We use the new
`llama_stack_api.internal` namespace for this.

In addition: the change moves kv/sql store impls, configs, and
dependency helpers under `core/storage`

## Testing

`pytest tests/unit/utils/test_authorized_sqlstore.py`, other existing CI

tests/unit/utils/inference/test_inference_store.py

@@ -9,8 +9,8 @@ import time
 import pytest
 
 from llama_stack.core.storage.datatypes import InferenceStoreReference, SqliteSqlStoreConfig
+from llama_stack.core.storage.sqlstore.sqlstore import register_sqlstore_backends
 from llama_stack.providers.utils.inference.inference_store import InferenceStore
-from llama_stack.providers.utils.sqlstore.sqlstore import register_sqlstore_backends
 from llama_stack_api import (
     OpenAIAssistantMessageParam,
     OpenAIChatCompletion,