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

src/llama_stack/providers/inline/files/localfs/files.py

@@ -13,11 +13,10 @@ from fastapi import Depends, File, Form, Response, UploadFile
 
 from llama_stack.core.datatypes import AccessRule
 from llama_stack.core.id_generation import generate_object_id
+from llama_stack.core.storage.sqlstore.authorized_sqlstore import AuthorizedSqlStore
+from llama_stack.core.storage.sqlstore.sqlstore import sqlstore_impl
 from llama_stack.log import get_logger
 from llama_stack.providers.utils.files.form_data import parse_expires_after
-from llama_stack.providers.utils.sqlstore.api import ColumnDefinition, ColumnType
-from llama_stack.providers.utils.sqlstore.authorized_sqlstore import AuthorizedSqlStore
-from llama_stack.providers.utils.sqlstore.sqlstore import sqlstore_impl
 from llama_stack_api import (
     ExpiresAfter,
     Files,
@@ -28,6 +27,7 @@ from llama_stack_api import (
     Order,
     ResourceNotFoundError,
 )
+from llama_stack_api.internal.sqlstore import ColumnDefinition, ColumnType
 
 from .config import LocalfsFilesImplConfig