[ADD] bus: add support for custom functions in imbus notify

This patch provides the possibility to implement a custom security layer on top
of Odoo's bus notification system which uses postgresql's NOTIFY command.

The key addition is the `ODOO_NOTIFY_FUNCTION` environment variable (opt-in),
which can now define a postgresql function to be called instead of the NOTIFY
command. This allows for greater flexibility and control over the notification
and triggering mechanisms within Odoo.

closes odoo/odoo#130368

Signed-off-by: Fabien Meghazi (fme) <fme@odoo.com>