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.

(cherry picked from commit 042c2ac)

Author: potiuk

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