Make find_user_from_sources extensible for EE

lib/api/api_guard.rb

@@ -42,7 +42,7 @@ module API
       include Gitlab::Auth::UserAuthFinders
 
       def find_current_user!
-        user = find_user_from_access_token || find_user_from_warden
+        user = find_user_from_sources
         return unless user
 
         forbidden!('User is blocked') unless Gitlab::UserAccess.new(user).allowed? && user.can?(:access_api)
@@ -50,6 +50,10 @@ module API
         user
       end
 
+      def find_user_from_sources
+        find_user_from_access_token || find_user_from_warden
+      end
+
       private
 
       # An array of scopes that were registered (using `allow_access_with_scope`)