Fix mypy typing errors by explicitly exporting Client (#643)

Resolves #567

Previously, importing Client from clickhouse_connect.driver would
fail mypy validation with "Module does not explicitly export attribute
Client". This occurred because the module didn't define __all__.

Added __all__ to explicitly export Client, AsyncClient, create_client,
and create_async_client, making these public API symbols clear to
type checkers.

Co-authored-by: Joe Spadola <joe.spadola@clickhouse.com>

Author: veeceey

clickhouse_connect/driver/__init__.py

@@ -11,6 +11,8 @@
 from clickhouse_connect.driver.httpclient import HttpClient
 from clickhouse_connect.driver.asyncclient import AsyncClient, DefaultThreadPoolExecutor, NEW_THREAD_POOL_EXECUTOR
 
+__all__ = ['Client', 'AsyncClient', 'create_client', 'create_async_client']
+
 
 # pylint: disable=too-many-arguments,too-many-locals,too-many-branches
 def create_client(*,