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/providers/agents/meta_reference/test_openai_responses.py

@@ -17,6 +17,7 @@ from openai.types.chat.chat_completion_chunk import (
 
 from llama_stack.core.access_control.access_control import default_policy
 from llama_stack.core.storage.datatypes import ResponsesStoreReference, SqliteSqlStoreConfig
+from llama_stack.core.storage.sqlstore.sqlstore import register_sqlstore_backends
 from llama_stack.providers.inline.agents.meta_reference.responses.openai_responses import (
     OpenAIResponsesImpl,
 )
@@ -24,7 +25,6 @@ from llama_stack.providers.utils.responses.responses_store import (
     ResponsesStore,
     _OpenAIResponseObjectWithInputAndMessages,
 )
-from llama_stack.providers.utils.sqlstore.sqlstore import register_sqlstore_backends
 from llama_stack_api.agents import Order
 from llama_stack_api.inference import (
     OpenAIAssistantMessageParam,