Expost init in client lightning api (#3005)

nvflare/client/lightning/__init__.py

@@ -35,6 +35,7 @@
     from nvflare.client import get_config as get_config
     from nvflare.client import get_job_id as get_job_id
     from nvflare.client import get_site_name as get_site_name
+    from nvflare.client import init as init
     from nvflare.client import is_running as is_running
     from nvflare.client import log as log
     from nvflare.client import receive as receive