Load providers configuration when gunicorn workers start (#38795)

When Gunicorn workers start, they do not fork, but start as new
Python interpreters and they should also perform loadig of the
configuration from installed providers.

airflow/www/gunicorn_config.py

@@ -36,5 +36,8 @@ def post_worker_init(_):
 def on_starting(server):
     from airflow.providers_manager import ProvidersManager
 
+    providers_manager = ProvidersManager()
+    # Load providers configuration before forking workers
+    providers_manager.initialize_providers_configuration()
     # Load providers before forking workers
-    ProvidersManager().connection_form_widgets
+    providers_manager.connection_form_widgets