[FIX] mail: messaging start when bus service is deployed

Before this commit, messaging.start() may crash due to bus service
not yet deployed.

This commit fixes the issue by ensuring `start()` is always called
when the bus service is deployed. To do so, we introduce a new
service (Messaging) that simply waits for bus service deployment
before invoking messaging start.

Task-2468483

closes odoo/odoo#66828

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>