FEATURE: add user_session_refreshed trigger (#9412)

Trigger an event for plugins to consume when a user session is refreshed.

This allows external auth to be notified about account activity, and be
able to take action such as use oauth refresh tokens to keep oauth
tokens valid.

lib/auth/default_current_user_provider.rb

@@ -148,6 +148,7 @@ class Auth::DefaultCurrentUserProvider
                                client_ip: @request.ip,
                                path: @env['REQUEST_PATH'])
           cookies[TOKEN_COOKIE] = cookie_hash(@user_token.unhashed_auth_token)
+          DiscourseEvent.trigger(:user_session_refreshed, user)
         end
       end
     end