feat: export StorageAdapter and get_storage_adapter at top level

The Storage Adapter spec (datajoint-docs#172) documents
``dj.StorageAdapter`` as the public base class and ``dj.get_storage_adapter()``
as the lookup helper, but they were not exposed via the top-level
namespace in 2.2.3 — plugin authors following the spec literally hit
``AttributeError: module 'datajoint' has no attribute 'StorageAdapter'``.

Adds the two public symbols to ``__init__.py``:

- ``StorageAdapter`` (ABC, defined in ``src/datajoint/storage_adapter.py``)
- ``get_storage_adapter(protocol)``

The private ``_discover_adapters`` helper is intentionally not exported.

Author: dimitri-yatsenko

src/datajoint/__init__.py

@@ -51,6 +51,9 @@
     "get_codec",
     "ObjectRef",
     "NpyRef",
+    # Storage Adapter API
+    "StorageAdapter",
+    "get_storage_adapter",
     # Other
     "errors",
     "migrate",
@@ -82,6 +85,7 @@
 from .instance import Instance, _ConfigProxy, _get_singleton_connection, _global_config, _check_thread_safe
 from .logging import logger
 from .objectref import ObjectRef
+from .storage_adapter import StorageAdapter, get_storage_adapter
 from .schemas import _Schema, VirtualModule, list_schemas, virtual_schema
 from .autopopulate import AutoPopulate
 from .jobs import Job