fix: the bug of async connection context manager (#1333)

- add `return` for `__enter__`

The buggy code didn't return the object, therefore it will always return
None within a context manager:

```python
with await lancedb.connect_async("./.lancedb") as db:
        # db is always None
```

(BTW, why not to design an async context manager?)

- add a unit test for Async connection context manager

- update return type of `AsyncConnection.open_table` to `AsyncTable`

Although type annotation doesn't affect the functionality, it is helpful
for IDEs.

python/python/tests/test_db.py

@@ -296,6 +296,13 @@ async def test_close(tmp_path):
         await db.table_names()
 
 
+@pytest.mark.asyncio
+async def test_context_manager(tmp_path):
+    with await lancedb.connect_async(tmp_path) as db:
+        assert db.is_open()
+    assert not db.is_open()
+
+
 @pytest.mark.asyncio
 async def test_create_mode_async(tmp_path):
     db = await lancedb.connect_async(tmp_path)