[FIX] google_calendar: use the event user token to send the request

Before this commit: it was possible to create an event for another user as
its organizer. But even if that user has a synchronized Google calendar, it
will be sent by the current user token, and it causes several issues.

The solution is to use the event's organizer token if it exists.

opw-3076595

closes odoo/odoo#110858

Signed-off-by: Pedram Bi Ria (pebr) <pebr@odoo.com>