ws: tighten CentralExchange API

This commit tigthen the public API for the `.CentralExchange` module,
so that it's clear which are the exported (public) functions, and
we do not leak the transport layer used by the central exchange

src/Gargantext/Core/AsyncUpdates/CentralExchange.hs

@@ -14,7 +14,10 @@ https://dev.sub.gargantext.org/#/share/Notes/187918
     
 -}
 
-module Gargantext.Core.AsyncUpdates.CentralExchange where
+module Gargantext.Core.AsyncUpdates.CentralExchange (
+    gServer
+  , notify
+  ) where
 
 import Control.Concurrent.Async qualified as Async
 import Control.Concurrent.STM.TChan qualified as TChan